From Test-Scratch-Wiki

The link to change one's signature.
Access the main home page of the Scratch forums. Scroll down to near the footer and click the blue link with the text "Change your signature". Alternatively, go to http://scratch.mit.edu/discuss/settings/USERNAME replacing "USERNAME" with one's username.

The page contains an editor which allows to one's signature to be changed. By entering text in to the box and clicking submit, a signature can be set. BBCode can be used. The green tick in the top right corner allows the new signature to be previewed, before actually being set.

See Also