how do you remix

Click on the ‘See inside’-button right above. Then you can edit the project. When you're ready click the ornge button right above that says ‘Remix’.
I hope you understand everything I wrote cause my English isn't so good. (I'm from Germany.)

Click on the see inside button. When you finished editing it, click on “Remix”. Remixes will not autosave until you press the remix button and revisit the page.
By the way, if you remix, there are some rules:
Give credit
No project copying (that is, making a remix but not changing anything at all)
Any project that is shared on the main scratch website allows remixing. Even those that say “DO NOT REMIX OR ELSE” all over them. This is because:

“By publishing your project on the Scratch website, you agree to license it under a Creative Commons Share Alike license. If you don’t want others to view and remix your creations, don’t share them on the Scratch website.” -Scratch FAQ, by the Lifelong Kindergarted Group at the MIT media lab (I assumed that the text on the website is also under the same license)

and Creative Commons Share Alike allows the remixing of projects as long as credit is given.

Your remixes must also be released under Creative Commons Share Alike Unported 3.0 or a compatible license, due to the complex legal stuff.