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+How Tos
+=======
+
+All of this equipment is here to help you get your work done more efficiently.
+However, learning how task-graphs are generated is probably not the work you
+are interested in doing.  This section should help you accomplish some of the
+more common changes to the task graph with minimal fuss.
+
+.. important::
+
+    If you cannot accomplish what you need with the information provided here,
+    please consider whether you can achieve your goal in a different way.
+    Perhaps something simpler would cost a bit more in compute time, but save
+    the much more expensive resource of developers' mental bandwidth.
+    Task-graph generation is already complex enough!
+
+    If you want to proceed, you may need to delve into the implementation of
+    task-graph generation.  The documentation and code are designed to help, as
+    are the authors - ``hg blame`` may help track down helpful people.
+
+    As you write your new transform or add a new kind, please consider the next
+    developer.  Where possible, make your change data-driven and general, so
+    that others can make a much smaller change.  Document the semantics of what
+    you are changing clearly, especially if it involves modifying a transform
+    schema.  And if you are adding complexity temporarily while making a
+    gradual transition, please open a new bug to remind yourself to remove the
+    complexity when the transition is complete.
+
+Hacking Task Graphs
+-------------------
+
+The recommended process for changing task graphs is this:
+
+1. Find a recent decision task on the project or branch you are working on,
+   and download its ``parameters.yml`` from the Task Inspector.  This file
+   contains all of the inputs to the task-graph generation process.  Its
+   contents are simple enough if you would like to modify it, and it is
+   documented in :doc:`parameters`.
+
+2. Run one of the ``mach taskgraph`` subcommands (see :doc:`taskgraph`) to
+   generate a baseline against which to measure your changes.  For example:
+
+   .. code-block:: none
+
+       ./mach taskgraph tasks --json -p parameters.yml > old-tasks.json
+
+3. Make your modifications under ``taskcluster/``.
+
+4. Run the same ``mach taskgraph`` command, sending the output to a new file,
+   and use ``diff`` to compare the old and new files.  Make sure your changes
+   have the desired effect and no undesirable side-effects.
+
+5. When you are satisfied with the changes, push them to try to ensure that the
+   modified tasks work as expected.
+
+Common Changes
+--------------
+
+Changing Test Characteristics
+.............................
+
+First, find the test description.  This will be in
+``taskcluster/ci/*/tests.yml``, for the appropriate kind (consult
+:doc:`kinds`).  You will find a YAML stanza for each test suite, and each
+stanza defines the test's characteristics.  For example, the ``chunks``
+property gives the number of chunks to run.  This can be specified as a simple
+integer if all platforms have the same chunk count, or it can be keyed by test
+platform.  For example:
+
+.. code-block:: yaml
+
+    chunks:
+        by-test-platform:
+            linux64/debug: 10
+            default: 8
+
+The full set of available properties is in
+``taskcluster/taskgraph/transform/tests/test_description.py``.  Some other
+commonly-modified properties are ``max-run-time`` (useful if tests are being
+killed for exceeding maxRunTime) and ``treeherder-symbol``.
+
+.. note::
+
+    Android tests are also chunked at the mozharness level, so you will need to
+    modify the relevant mozharness config, as well.
+
+Adding a Test Suite
+...................
+
+To add a new test suite, you will need to know the proper mozharness invocation
+for that suite, and which kind it fits into (consult :doc:`kinds`).
+
+Add a new stanza to ``taskcluster/ci/<kind>/tests.yml``, copying from the other
+stanzas in that file.  The meanings should be clear, but authoritative
+documentation is in
+``taskcluster/taskgraph/transform/tests/test_description.py`` should you need
+it.  The stanza name is the name by which the test will be referenced in try
+syntax.
+
+Add your new test to a test set in ``test-sets.yml`` in the same directory.  If
+the test should only run on a limited set of platforms, you may need to define
+a new test set and reference that from the appropriate platforms in
+``test-platforms.yml``.  If you do so, include some helpful comments in
+``test-sets.yml`` for the next person.
+
+Greening Up a New Test
+......................
+
+When a test is not yet reliably green, configuration for that test should not
+be landed on integration branches.  Of course, you can control where the
+configuration is landed!  For many cases, it is easiest to green up a test in
+try: push the configuration to run the test to try along with your work to fix
+the remaining test failures.
+
+When working with a group, check out a "twig" repository to share among your
+group, and land the test configuration in that repository.  Once the test is
+green, merge to an integration branch and the test will begin running there as
+well.
+
+Adding a New Task
+.................
+
+If you are adding a new task that is not a test suite, there are a number of
+options.  A few questions to consider:
+
+ * Is this a new build platform or variant that will produce an artifact to
+   be run through the usual test suites?
+
+ * Does this task depend on other tasks?  Do other tasks depend on it?
+
+ * Is this one of a few related tasks, or will you need to generate a large
+   set of tasks using some programmatic means (for example, chunking)?
+
+ * How is the task actually excuted?  Mozharness?  Mach?
+
+ * What kind of environment does the task require?
+
+Armed with that information, you can choose among a few options for
+implementing this new task.  Try to choose the simplest solution that will
+satisfy your near-term needs.  Since this is all implemented in-tree, it
+is not difficult to refactor later when you need more generality.
+
+Existing Kind
+`````````````
+
+The simplest option is to add your task to an existing kind.  This is most
+practical when the task "makes sense" as part of that kind -- for example, if
+your task is building an installer for a new platform using mozharness scripts
+similar to the existing build tasks, it makes most sense to add your task to
+the ``build`` kind.  If you need some additional functionality in the kind,
+it's OK to modify the implementation as necessary, as long as the modification
+is complete and useful to the next developer to come along.
+
+New Kind
+````````
+
+The next option to consider is adding a new kind.  A distinct kind gives you
+some isolation from other task types, which can be nice if you are adding an
+experimental kind of task.
+
+Kinds can range in complexity.  The simplest sort of kind uses the
+``TransformTask`` implementation to read a list of jobs from the ``jobs`` key,
+and applies the standard ``job`` and ``task`` transforms:
+
+.. code-block:: yaml
+
+    implementation: taskgraph.task.transform:TransformTask
+    transforms:
+       - taskgraph.transforms.job:transforms
+       - taskgraph.transforms.task:transforms
+    jobs:
+       - ..your job description here..
+
+Custom Kind Implementation
+``````````````````````````
+
+If your task depends on other tasks, then the decision of which tasks to create
+may require some code.  For example, the ``upload-symbols`` kind iterates over
+the builds in the graph, generating a task for each one.  This specific
+post-build behavior is implemented in the general
+``taskgraph.task.post_build:PostBuildTask`` kind implementation.  If your task
+needs something more purpose-specific, then it may be time to write a new kind
+implementation.
+
+Custom Transforms
+`````````````````
+
+If your task needs to create many tasks from a single description, for example
+to implement chunking, it is time to implement some custom transforms.  Ideally
+those transforms will produce job descriptions, so you can use the existing ``job``
+and ``task`` transforms:
+
+.. code-block:: yaml
+
+    transforms:
+       - taskgraph.transforms.my_stuff:transforms
+       - taskgraph.transforms.job:transforms
+       - taskgraph.transforms.task:transforms
+
+Similarly, if you need to include dynamic task defaults -- perhaps some feature
+is only available in level-3 repositories, or on specific projects -- then
+custom transforms are the appropriate tool.  Try to keep transforms simple,
+single-purpose and well-documented!
+
+Custom Run-Using
+````````````````
+
+If the way your task is executed is unique (so, not a mach command or
+mozharness invocation), you can add a new implementation of the job
+description's "run" section.  Before you do this, consider that it might be a
+better investment to modify your task to support invocation via mozharness or
+mach, instead.  If this is not possible, then adding a new file in
+``taskcluster/taskgraph/transforms/jobs`` with a structure similar to its peers
+will make the new run-using option available for job descriptions.
+
+Something Else?
+...............
+
+If you make another change not described here that turns out to be simple or
+common, please include an update to this file in your patch.