E.g. follow this link. Fill the title, and a short description if necessary. If there is a corresponding CMSSW issue, include a link to it in the issue description (in this case no additional content is necessary, i.e. the link alone is fine; you can also use the same title for the issue).
If it makes sense to create an initial plan for an activity, do that by including a task list, with one element for each task
The activity can be assigned to a developer already at the creation phase. There is a quick link to assign the activity to yourself.
Add the Activity label for the issue
Then click Submit new issue. GitHub will automatically add the new issue to the Activities project.
The issue can also be created through the Activities project. For example, in the By importance view, start writing the the title of the issue, and then click Create new issue
