For cowled two-strokes, where it's awkward to hold a finger over the carb and still be able to turn the prop over, get a rubber stopper from the hardware section at Home Depot. They come in a range of sizes. Just open the throttle, plug the throat, turn maybe five cranks--until you hear the squishing sound. Then remove the stopper, throttle back, glow on, do the bump test if you like, then backflip and you're off.