When I first started wearing I always put them on laying down as it felt the most natural to what I knew of diapers.
I wouldn't call myself a real big guy but I'm kind of "thick"....having said that, we all look thinner in the waist when laying as gravity takes course. So after putting on, and entering back to standing position, the diaper always felt too loose. Looking back I don't think I was taping properly but still attribute at least some of it to what I explained above.
Years and years later, I can perfect almost any position but find it best when sitting or standing. 95% of the time I do the sitting method on a toilet (ironic, right?). Bottoms tapes secure, then sometimes stand to make sure I'm getting a good fit on the top tapes/above the waist.
I've also when standing, set the diaper unfolded between my underwear for stability and kind of pulled everything up at once. Your legs have to be spread or else there's too much messing with leg gathers and other....anatomical annoyances.
