When uploading a remix, you are prompted to give credit to the original creator of the project in the "Notes and Credits" until something is typed in the box. It is also required that you change an aspect of the project whether it be the themes, graphics or scripts. Remember that remixes that do not change anything or do not give credit to the original author can be reported.
To remix a project, click the green Remix button next to the See inside button in the top right corner of the project page. Alternately, on the project page click See Inside and in the top-right of the project editor, click on the Remix button to upload the project as a remix from your account.