Find an issue - often this happens on a local Sakai environment at your institution.
Replicate the issue, ask:
Can it be reproduced consistently?
Are specific conditions needed to trigger it? Identify them if so.
Does it affect multiple Sakai tools? If yes, which ones?
Examples: Date picker and Rich-Text Editor are in many tools
Also, take note of steps to reproduce - you will need these later!
Replicate the issue elsewhere.
Try different nightly servers. If it happens on Trunk (master), does it also happen on Sakai 12 Jira Testing server? If it seems new, test back a version! Does it happen on Sakai 11.x maintenance?
Some issues require specific Sakai properties to be set - if something appears to be missing, ask in Slack or the sakai-dev mailing list if the necessary properties are present on the server where you need to test.