Watch the first ~40 minutes of this video on preregistration by the Center for Open Science for an introduction to the process.
Make a copy of the OSF preregistration google doc and change from editing to suggesting mode. Share the document with everyone on the study team.
This way, when you add things, they'll highlight in green and we can discuss them.
You'll be asked for the following information.
Explanation of existing data
Data collection procedures
Sample size rationale
Share your preregistration draft with the study team. Formally, you'll submit a link to your draft (with your pull request) before deploying your experiment. But it's great to share it early so we can discuss.
Submit the final approved preregistration on the OSF: https://osf.io/prereg/.