Try cutting it with the backing paper still on - once you get a good fit, peel off the backing paper and KEEP it! Then use it as a stencil for your next change. Just take a pen, trace it onto the new bag, then cut the new bag. Just keep a eye on your stoma, because it will change sizes as you heal.
Don't worry, you will get used to it, and it will get easier. It's hard for everyone in the beginning!
As for itching, it's horrible, and there isn't a whole lot to do for it. If it isn't that you need a bag changing, the best relief I could ever get was to push on it where it itched (gently). That, and to distract myself! Ugh! :)
If at first you do not succeed, then skydiving is surely not meant for you.
Jo - UC, total colectomy and ileostomy on August 24, 2008.