Find a hill, use the slope as the in-run. It's easiest to build a ladder style in-run for the least amount of materials. Use carpeting (preferably astro turf) and cover the rungs of the ladder, and boom. Or put two 2x4's and screw plywood over the top to make a raised surface and put Astroturf over that. Make the lip of your drop-in a solid surface, no ladder. Just makes it easier when actually hitting it.