| The instructions for iron-on decals usually tell you to put the shirt (or whatever you're applying the decal to) on a very firm surface, preferably your formica kitchen counter. I've always done that with good results.
But I recently wanted to apply a decal to the back of a t-shirt that had a pocket on the front. There was no way to get a completely flat surface where I wanted it without the pocket creating bumps through to the back.
My solution was to go to a hardware (Home Depot) and buy a 12" x 12" adobe floor tile... one that was unglazed and absolutely flat (no ripples). I slipped that into the shirt to use as the 'formica.' Worked super!! The size was slightly larger than any decal I'd be applying (max. size 8-1/2" x 11"). And I got a little tile to slip into pockets when I want to put a decal on them, or to use when applying decals to kids' shirt sleeves, pant legs, etc.
The cost of the large tile was only $1.38... can't beat that!
No doubt a glazed tile would work just as well. I just happened to pick unglazed. Just be sure the tile you buy is perfectly smooth. | 
