Don't let the word "Legitimate" scare you. This is for any upload. I'm not quite sure what they classify as illegitimate.
We need to upload three items to the Steam workshop. You can do almost all of this in-game. The only requirement is that you need to have three different images on your actual computer. It doesn't matter what they are. This is because you need to upload a thumbnail along with your 'project'. Your 'project' can just be an exact duplicate of one of the pre-made games.
So, load up any game you want. Press the 'modding' tab on the top of the screen. Press workshop upload. Then press the browse files button (yellow folder) in the "Workshop Thumbnail URL" section. Then, upload one of your images. Afterwards, press upload. That's 1/3. You can repeat this exact process to get the other two.
Once you upload three items to the steam workshop, you'll get this one.