What i do is take the original sprite and paste it in a program whose canvas size conforms to the size of an image pasted / is easy to change (Paint); I then crop the image down so that the very edges of the original sprite are touching the canvas border...this isnt required, its just so you don't have to work with uneccessarily larger images.

Anywhoo, select the entire image and enlarge the overall size by 200%; copy this, then repaste it onto that same file. Find out what the dimensions of the image are (since Graphics Gale uses preset, non-conforming canvas sizing); start a new file in Gale withose dimensions. Paste it and then lower the opacity down to around twenty percent. Add a new layer, and trace over the enlarged original sprite in that new layer. Try to make it look as similar as possible, but some things can be altered or added if it would make it look better in a larger pixel art size.

That's just my method, though, but you do still need to enlarge the original, find a way to trace over it, then copy the shading as closely as possible. That's pretty much all there is to it, actually.