I pick a place where I want to sit for an hour and cover an ottoman with the large garbage bag.
Apply to the skin with a non-metal tool like the back of a plastic spoon, a paintbrush or a long-handled wooden spoon. (It’s easier for someone else to do it for you.) Apply until the area is thickly covered, like frosting a cake or plastering a wall. Leave on for 5 - 6 hours. It’s messy. I let it dry, then I use the plastic grocery bags on my feet to walk around (carefully). Some people bandage the hennaed area, which I have not tried.
To get the six hours, I put an old towel on the bed and leave my foot out from the covers for the night. You will get crumbs of dried henna on the bed but if it’s dry when you go to bed, it won’t stain. I wasn’t harmed by leaving it on if I slept longer than six hours. The dog did not lick it off.