Under the Scratch logo, there's a bar that says "Scratch Forums". On the blue bar under that, there are three items: "Forums Home," "Search," and "Profile." Click on Profile, and on the left there will be a sidebar that says "Profile Menu". Inside it, click "Personality," which will give you the page for your signature! Just fill out the box (which will turn into your signature) and then press submit at the bottom.
You need 50 posts, though. But you have 90, so you'll be fine.
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