Check first Pixote' tutorial.
http://falloutmods.wikia.com/wiki/Building_New_Critters_for_FalloutWhile this tutorial is very informative, i developed my own workflow in Photoshop.
I found out that making strokes with a eraser is simpler and faster than marking elements with lasso tool and copypasting it.
It does mean: using 2 shortcuts, one precise stroke with mouse, two rough ones, one custom shortcut for indexing image, tapping enter twice, one shortcut for saving, tapping enter, closing shortcut.
Making 10 seconds for a single frame.