According to the TC39 process document, to advance from Stage 2 to Stage 3, reviewers and the editor group must sign off on the spec text. This document describes the Stage 3 review process.
Reviewers for Stage 3 volunteer as such in TC39 meetings when discussing a proposal. If the proposal is ready, the champion can ask the committee for reviewers at the same meeting as it advances to Stage 2. Sizable proposals are encouraged to have multiple reviewers. Different reviewers can provide different perspectives.
Reviewer feedback is tracked in a GitHub issue in the proposal repository (example). It may also be useful to send out a reminder email ahead of the meeting to the reviewers.
When reviewing a TC39 proposal, read the specification text, README and other documentation carefully. Some questions which may be useful to consider in a review:
To give your feedback to the champion group, file issues in GitHub, or make pull requests with proposed changes.
Reviews are expected to be performed at least two weeks in advance of the TC39 meeting.