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The first step to making a mockup is to open the DevTools. This can be done with the keyboard shortcuts ctrl + shift + i or F12.
This DevTools menu allows you to do many cool things, such as create new elements, and modify existing ones. In all browsers(that I know of) you can use a little selector at the top-left of the DevTools panel to enable a mode that if you click on an area of the screen, it brings you to the element in question within the DevTools panel.
There's not much else advice I can give you. Just look around the Scratch site to get inspiration, and then either create or edit an element on a page you like to fit your needs.