I am not 100% sure, but I think what you are asking about is the pratice of moving the emulsion around on a piece of sx-70 film before the emulsion has set. You could use a variety of tools, homemade or otherwise and just scratch around (for the want of a better word). It's fun, but as all "simple"things is harder than one can image to create wonderful one of a kind images.
After taking the picture, just take a sharp tool and play with the emulsion. It is as simple as that. Be sure the tool is not too sharp as the scratch all the way to the base, unless that is the effect you are looking for. This is like playing in the sandbox, nothing is really wrong with whatever you decide.