Update guidance

Author: maxrjones

.github/ISSUE_TEMPLATE/release-checklist.md

@@ -18,8 +18,8 @@ assignees: ''
 
 - [ ] Check [SPEC 0](https://scientific-python.org/specs/spec-0000/#support-window) to see if the minimum supported version of Python or NumPy needs bumping.
 - [ ] Verify that the latest CI workflows on `main` are passing: [Tests](https://github.com/zarr-developers/zarr-python/actions/workflows/test.yml), [GPU Tests](https://github.com/zarr-developers/zarr-python/actions/workflows/gpu_test.yml), [Hypothesis](https://github.com/zarr-developers/zarr-python/actions/workflows/hypothesis.yaml), [Docs](https://github.com/zarr-developers/zarr-python/actions/workflows/docs.yml), [Lint](https://github.com/zarr-developers/zarr-python/actions/workflows/lint.yml), [Wheels](https://github.com/zarr-developers/zarr-python/actions/workflows/releases.yml).
-- [ ] Run the ["Downstream" workflow](https://github.com/zarr-developers/zarr-python/actions/workflows/downstream.yml) and verify Xarray and numcodecs tests pass.
-- [ ] Run the ["Prepare release" workflow](https://github.com/zarr-developers/zarr-python/actions/workflows/prepare_release.yml) with the target version. This will build the changelog and open a release PR.
+- [ ] Run the ["Prepare release" workflow](https://github.com/zarr-developers/zarr-python/actions/workflows/prepare_release.yml) with the target version. This will build the changelog and open a release PR with the `run-downstream` label.
+- [ ] Verify that the [downstream tests](https://github.com/zarr-developers/zarr-python/actions/workflows/downstream.yml) (triggered automatically by the `run-downstream` label) pass on the release PR.
 - [ ] Review the release PR and verify the changelog in `docs/release-notes.md` looks correct.
 - [ ] Merge the release PR.
 

docs/contributing.md

@@ -316,6 +316,16 @@ If an existing Zarr format version changes, or a new version of the Zarr format
 Open an issue on GitHub announcing the release using the release checklist template:
 [https://github.com/zarr-developers/zarr-python/issues/new?template=release-checklist.md](https://github.com/zarr-developers/zarr-python/issues/new?template=release-checklist.md). The release checklist includes all steps necessary for the release.
 
+### Preparing a release
+
+Releases are prepared using the ["Prepare release notes"](https://github.com/zarr-developers/zarr-python/actions/workflows/prepare_release.yml) workflow. To run it:
+
+1. Go to the [workflow page](https://github.com/zarr-developers/zarr-python/actions/workflows/prepare_release.yml) and click "Run workflow".
+2. Enter the release version (e.g. `3.2.0`) and the target branch (defaults to `main`).
+3. The workflow will run `towncrier build` to render the changelog, remove consumed fragments from `changes/`, and open a pull request on the `release/v<version>` branch.
+4. The release PR is automatically labeled `run-downstream`, which triggers the [downstream test workflow](https://github.com/zarr-developers/zarr-python/actions/workflows/downstream.yml) to run Xarray and numcodecs integration tests against the release branch.
+5. Review the rendered changelog in `docs/release-notes.md` and verify downstream tests pass before merging.
+
 ## Benchmarks
 
 Zarr uses [pytest-benchmark](https://pytest-benchmark.readthedocs.io/en/latest/) for running