Almost all custom producers will use embroidery to customize baseball hats.  Screen printing is possible but fiddly and expensive, with little benefit.  The level of detail attainable from embroidery is usually sufficient.

What materials are available?

Cotton/ Twill

A textile weave using cotton which tends to hold its shape fairly well.

Wool

Can be acrylic or wool blend, or a polyester/ cotton/ spandex blend.

Mesh

A loosely woven material that makes it lighter and breathable.  It’s often fused with buckram which is a stiff cotton or linen used to stiffen the hat.

Visor

Usually plastic which is sewn inside the fabric.

Sweatband

Cotton and elastic.

Closure

The closure is at the back of the hat and is either adjustable or fixed width.  An adjustable closure can be made from either Velcro, adjustable tuck strap or plastic snap closure.

Eyelets

Eyelets are the 2-6 small holes over the hat which allows airflow to the head.  They are made from either metal or reinforced the stitching.

How many panels should I get?

Custom Baseball hats usually come as 5 or 6 panels.  If 5 panels, the front of the hat will have just one panel the whole way across, similar to a trucker hat.  If there are 6 panels, the front of the hat, above the brim will be broken up into two panels.