Set the title, Description, BPM, and (optionally) relevant tags
Important: upload an mp3 preview of your remix. Alternate downloadable files can be added later (see "Edit Files")
Check the boxes next to your source tracks. You can filter the list of sources by any string (artist, title..) as many times as you like, and the selected sources will be retained.
Click "Go to Step 2". On the second page, you can be more specific about which stems/parts of your sources you used.
Type in the names of any non-MixterPlus contributors to your remix, and click "Save Tree"