There are many different ways to get rid of any existing old grass, weeds….

Or vegetation. You can rototill the area thoroughly 6-8” deep. Be careful of existing irrigation. This will work in. Digging up any existing vegetation will expose many weed seeds. When done, remove any weeds, rocks, or debris that have been turned up. Do a rough level. Because you will be amending and rototilling again, a rough grade will do.