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Go to the project issues Board, then take the issue you want to review. Be sure don't review one of your MR, the main usage of code review is to have a look and comment code done by other developers.
When you choose the issue you want to review, move it to reviewing and open the Merge Request page associated.
The main things to check are the following:
If you see something, use the Merge Request to make feedback to the code author. If it's about a precise line or method, comment it in the diff page.
Else open a discussion inside the discussion part. Prefer discussion than commit, because a discussion must be mark as resolved to allow the merge request to be merged.
So if you write some feedbacks, when you're done, assign the Merge Request to the code author. Let him now you add some comments to his code. And finally move the related issue to TODO in the project issues Board.
Have a last look to be sure you didn't forgot something.
Note: Only masters are allowed to merge. If you haven't permission, then assign the MR to a master.