To remix, download the project, make some changes (important step) and upload it again.
To make a gallery, click the "galleries" tab at the top of the page and click the link that says "create a new gallery".
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As hmnwilson says, remixing is accomplished by downloading, changing, then uploading a given project; to create a gallery, click the blue underlined "create" in the gallery box on your home page thingy.
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remix by downloading teh project then uploading it to scratch web.
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