Hi, my name is Marsha, and I've had my ileostomy for over 50 years. Although that was a long time ago, I still remember the trial and error applications, leaks, and shortage of "good supplies". Today you're lucky to have stoma therapists available, and if you don't have one, it's relatively easy to find one. Another option is to contact other ostomy supply companies and ask for samples...
I'm not sure what product you're using. I much prefer a two-piece appliance because of the ability to place the wafer more accurately. I use Convatec moldable wafer and pouch. That comes in a flat wafer and a convex since I have a very uneven belly surface...
The most important issue besides fitting a wafer is treatment of your skin. I gave up using soap under the wafer years ago, and for that matter, I don't shower with my appliance off. Soap or body lotion residue impedes sealing the wafer and can cause leaks. I was told by some young stoma therapists that I was using the products in the "wrong order". I didn't argue, but I tested the damn products back in the day!
I use a solvent remover (Smith and Nephew) to get off old adhesive. Next, I use Safe and Simple peristomal cleaner wipes. Although it's duplicating the Smith and Nephew cleaner, I find that this one really cleans up any leftover residue. Next, I wash the skin with hot water followed by cold water and dry well. I then use Smith and Nephew Reg (as opposed to no sting) skin prep. The sting makes me feel like I'm "healing" whatever abrasions I might have around the stoma. I then dust the area around the stoma with stoma adhesive powder. (This is what the therapists thought should come before the skin prep. I humbly disagree.) Stoma adhesive powder has wonderful healing elements in it, and once I place the wafer around and on the skin, I can add additional powder to any gaps around the stoma. If I'm not active, changing takes 5-10 minutes. If active, good luck. For me, the best time to change is first thing in the morning before eating or in the evening before dinner. After the wafer is on, you can follow the directions to attach the pouch...
Everyone develops a procedure that works for them. Back when I was a "kid", I found that taking baths shortened the time my wafer stuck, so I rarely take baths. And while I love to shower without the wafer and pouch on, I don't like to expose my skin to the softeners in soaps, so I leave my appliance on when showering. If it's a day I have to change, I'll take it off in the shower and just let the hot water do its work. Just a simple luxury. The best advice to you is to try different products to see which ones work best for you. Best of luck. Feel free to write if you need more suggestions. Marsha