this is something that i do to help make it easier. I have four boxes, the kind that the bags come in, and in each box i have everything i need for a change. I have a disposal bag, a tube of paste, my little towel squares to clean/dry with, a precut wafer and a bag, that i already closed (you have no idea how many times i put a new bag on and forgot to close it!)
I think that makes it easier, cause you dont' have to rush around and worry about forgetting something. I also have a waterproof mat that i lay on, so if i shoudl make a mess, it doesn't go anywhere but on there, that gives me some peace of mind.
As for the itchies, i HATE those, but i have no advice except what they told you already.
You can use the little measure thing that comes with your wafers to measure over your stoma first, then put that on the wafer and cut it out, i do agree that you should leave the backing on.
LIke they said, it's ok to have a little residue, the most important thing is that it is completly dry. You may be having itching cause you aren't getting the wafer all the way dry after you shower with it on. If i dont' get good and dry after a shower, i itch like crazy.