It's very easy. Cut the scrap ABS up with tin snips as small as possible - bigger pieces just take longer to disslove. Drop them into a sealable container - I use an old metal acetone tin (don't use plastic , or glass - for some reason it doesn't keep as well). Cover with just enough acetone, and let it sit for a few days. Keep an eye on it and top up with acetone if it gets too thick. You can use it at just about any consistency, but works best for glueing armour when it is runny but is a solid colour. If it's needed for filling holes or cracks, let it get a little thicker. Keep adding ABS and acetone as you need it.