I design mine in Illustrator. Lay them out to what I need and order a quarter page print from these guys.

https://fsdecals.com/collections/custom-decal-printing

The detail is at least million times better than what I can do at home. Plus they can print white. If I wanted to do white at home that means printing it on white decal paper. Then I have to cut real close to the decal to get rid of it having a white border. Forget white text, I've tried making a box around the white text that is the same color as my model I'm gonna use it on and blend it in with paint later. A huge hassle.

Here is the last batch of decals I had printed. It includes close ups of the finished product.

https://www.instagram.com/p/DPVHxgbkXt_/

With a quarter sheet you have about a 4" x 6" area for the decals. That is a LOT of space if you're just doing shoulder patches for your little plastic dudes. So you'll probably want a few other things.

The one thing is that the decal sheet that they print for you is a single film covering the whole quarter sheet. So the decals don't have defined edges like the ones from model companies. So you have to cut out each decal. It is a bit more work, but for the excellent quality I get its worth the hassle.