Example of a brush on a gif:

◆ First

▹Obviously, choose your gif that you want to use and open it in photoshop. 

▹Open a new layer and make sure that it's at the top also make sure you're in the first frame.

◆ Second

▹choose any brush(es) you want to use and brush it on your new layer.

and you're done!

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Shoutout to glueckschmerz for requesting this tutorial. 

Thanks for reading!