Some designs aren't suitable for stencils.
Don't tell me you can't draw or paint freehand. I'm not inclined to believe you. If you don't like how your first shirt turns out, keep practicing. The thrift store around here sells plain t-shirts for a dollar each. Textile medium costs a dollar at my craft store. The acrylic paint I bought is $5.50 per tube, but they could paint hundreds of t-shirts.
Hopefully at least one of these techniques appeals to some of you and you're inclined to try liberating yourself from the stencil.
Step 1: Gather and Prepare the Materials
cardboard or something else thin and stiff that fits in your t-shirt
folded paper towel (or other absorbent material)
water resistant paper plate or other item to use as a palette
If your t-shirt is new, it's a good idea to wash it. Sometimes new shirts have a substance on them called sizing that hinders paint absorption.
Cut out your cardboard to fit the inside of your shirt. I used some old pizza boxes for mine. Cover the cardboard with waxed paper and tape the waxed paper in place. Slip the covered cardboard inside your shirt (the one you plan to paint, not the one you're wearing).
Arrange your paint, cup of water, brushes, palette, and textile medium so they're easy to reach.
My bottle of textile medium says to mix 2 parts medium with 1 part acrylic paint. I estimate. The textile medium makes the acrylic paint more flexible so it doesn't stiffen the fabric. This makes it resistant to flaking off in some parts; acrylic paint already does a good job of permanently staining shirts. Sometimes I mix the textile medium with the primary colors first. Sometimes I mix it after I've mixed a color I want to use. Either way works.