Sit on the toilet. Spread your knees farther apart than you usually might, so that you have maximum access and visibility while you figure this out. Or you can squat and sit like a frog on the toilet seat. I find that the easiest, painless way.
Alternatively, you can stand to insert a tampon, placing one foot on a higher surface such as a toilet seat. If this works better for you, give it a shot. However, most women prefer sitting on the toilet so that any stray bleeding is contained.