You can wait a bit if you need to. It will just burn some oil and maybe foul the plugs. You can do the job without compressed air if you don't have access to that. You can feed a small diameter rope into the cylinder through the spark plug hole and MANUALLY turn over the engine by hand so the piston pushes the rope up against the valves, holding them closed. It takes a bit longer but works fine as long as you keep one end of the rope out where you can easily remove it. I tie a big nut to the end so it won't accidentally fall into the cylinder.
