1. Find a pony wearing the item you want to try on. Does not need to be a pony you own. In the URL for that pony's page there is a phrase "pet=" followed by a number. You'll want that number.
For my example use this Pony (she's not actually wearing items, sorry, I just needed an example URL so you can see where things go): http://www.ponyisland.net/index.php?src=pets&sub=profile&pet=8221655&usr=285193
The part I'll take is: 8221655
2. Take the pony you want to try the items on and copy their url. What you need to do is get their long URL which holds all their hex and gene info. The best way to do this is to go to the breed function of jwebgen, plug your pony in as both mother and father, select their genes, and select adult. The test breed baby will be an exact clone of your pony. Its image should have something similar for a URL.
But with your pony's hexes and genes...of course.
3. Take the end part of that URL. Everything from breed back. You should have a string like this:
4. Take the following URL. Where there are bracketed areas, replace them with whatever is specified in the brackets.
http://ponyisland.net/image.php?type=pet&pet=[ID# of pony wearing items]&size=300&[2nd half of Gen URL of pony to try items on]
So, using my two example ponies. I'll end up with this as a URL.
5. Paste the resulting URL into your browser and you'll get a picture of your pony wearing whatever items the other pony is. It also transfers tattoos so you'll probably want to find an untattooed pony wearing only the item you want to try on.
Also, the pony wearing the items does NOT need to be the same breed or gender as the pony you're trying things on. And the pony you're trying items on can still be a baby since you'll "grow" it using the test breed gen.