Use a sheet of metal (like an old sign, etc), spread a thin layer of grease over the metal. After each shot, refresh the grease with a pass of a paint roller.
Or, any source of cheap sheets of paper (I use flip chart paper). With paper, make sure to have a solid backstop like plywood.
Never mind what someone or some book tells you about choke patterns - every individual shotgun will behave differently with different choke tubes and/or different shells.
Ideally, you want to try at least 2 shots with each brand of ammo through each of your choke tubes, at each distance. Start at 16 yards, back up to 20, 25, 30, 40, and possibly 50 yards, if desired. At each distance, take 2 shots through each choke with each brand. If you're using paper, write down the distance, choke setting, and shell brand on each target so, when you get back home, you can review what you saw at the range.
Choke performance has as much to do with the shot cup in the shells as it does with the choke tube selection. This is why I say, use multiple brands of shells when patterning. Trust me, you won't need a book to tell you which patterns leave gaping holes birds can escape through, which are just right (at a given distance), and which will result in hamburger.
