like cece said laying on your back does help to get your stomach flatter adn make it easier to put on. Another thing is, since your surgery is new, your stomach is probably still swollen and that makes it harder.
This is how i put mine on, maybe it will help, i use the hollister new image.
I get my stuff ready, take the old bag off, wipe it down with a square of towel to make sure it is dry. I cover the stoma with another square of cloth, lie down on the floor grab the wafer and quickly remove the cloth place the wafer and cover with a clean cloth. I put the bag on, then i use my hands and rub them over the wafer a bunch, smoothing it down. When i get up, i get the hairdryer and i use it on the wafer for a little bit, not long just enough to make the thing sort of melt on me, lol, not the best way to explain it.
I use the paste, most everyone else uses the eakin seals.
If you are still using the wipes they gave you to protect your skin, or if you are using the powder that can shorten wear time, as can certain soaps, and creams