Shirt must be 50% polyester or more. At 50% it's still going to look "distressed" or faded. There's nothing we can do about this, it's just science. Polyester fabric has pores that open at high temperature that allows the sublimation ink to enter. The less polyester, the less open pores, and the less ink transferring to the shirt.
70% or more polyester is recommended. 90% is ideal. If you want a bright and vibrant shirt, then use 90% or 100% polyester. It's that simple.
You MUST use a heat press
Sorry, but an iron won't work. Not even if you're really careful and you press really hard.
Shirt must be light color
White shirts are best. Keep in mind, sublimation is an ink transfer, so it's pretty much like writing with an ink pen on a piece of paper. All ink colors look good on white, cream, and light gray.