The first thing you would do is take a screenshot of the game, in whatever emulator you happen to use. You would then crop everything around a tile, and paste it into a chipset. Grab as many chips as you wish, then go from there.

That's really all there is to it, but it always takes a long time. Not only that, but some games don't use the same dimensions for tiles, so you'll have to adjust that as well - which makes the whole process tedious and unrewarding, since it won't look quite right.