didn't i see you asking a similar question elsewhere? i'd never heard of "plastic pockets" before.
in doing some keyword searching, i found this link about sewing transparent pockets onto shirts.
Craft: Plastic Pocket T-Shirt | Crafts | Spoonful
was that what you wanted to do? heat press clear printed pockets onto shirts? if you wanted to do that, unless you're talking about physically melting the plastic itself onto a shirt, i would question the durability.
i would think the best way to do it is to try and look for 2 sided iron on adhesive tape or something to that effect, but it would leave ugly blotches on the inside that show through clear plastic.
it's such a specialized thing that i'd bet whoever does it created their own process DIY style.