For each release, some steps to check:

start: declare the new Language Versions when a common branch becomes available (this has a checklist of its own)
start: review/triage issues that should be considered during that milestone (scrub unlabeled issues)
preview: review public API changes
preview: remove any unused Language Version
preview/RTM: notify partners of preview and RTM packages being published to NuGet
preview/RTM: notify documentation team of first preview and RTM timeline (to publish docs)
preview/RTM: update Visual Studio release notes
RTM: announce the new features on the team blog and in the Visual Studio Preview release notes
RTM: record the language history and move the language proposals to the appropriate folder
update the package description(s)

For each language feature:

work with LDM to identify a champion and define the feature
update the Language Feature Status page as we start working on features
identify a designated reviewer
notify Jared and Neal on PRs with public API changes
breaking changes need to be approved by the compat council and documented
designated reviewer should document and validate a test plan
blocking issues should be identified and resolved before merging feature back to master (that includes resolving PROTOTYPE comments)
update the feature status when the feature is complete

To add a language version:

add it to the compiler (tests have a checklist)
add it to project-systems so that it appears in the UI drop-down
for VB, add it in an internal repo for legacy project system
add it to release.json (needs to be confirmed)

Project Overview

Project Roadmap

Labels used for Issues

Performance considerations for large solutions

Other documentation in the repo

Contributors

Building, Testing, and Debugging

Contributing Code

Tool Authors

Getting Started

Samples and Walkthroughs

Status

Language feature status

Edit-and-Continue Supported Edits

.NET Core

Runtime Code Generation

Interactive and Scripting

Interactive Window Documentation

Scripting API Samples

C# Interactive Walkthrough

Testing

How to Investigate CI Test Failures