See the "Design invariants" section in the new linter doc. This guidance applies for formatters as well.
We generally avoid offering users two different formatters for the same language. It might be okay if the formatters have different strictness (like gofmt vs gofumpt) or if they format different language dialects (each Hashicorp Config Language tool has a different formatter). Note that the opposite is not true: a formatting tool like Prettier supports multiple languages.
Start in format/private/formatter_binary.bzl.
This has some dictionaries that define the tool used for each language.
format/private/format.sh may also need a small change to wire up the file extensions applicable to this tool.
Run format/private/mirror_linguist_languages.sh to get a "canonical" list of extensions used for this language.
In the example folder, add source files that demonstrate incorrect formatting and verify that the bazel run format command corrects it.
Add a new test case in format/test/format_test.bats so that we have some automated testing that the formatter works.
Update the README.md to include your formatter in the table.