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.
Step 2: Sketch an outline
I take my crayon drawings to the table when I'm ready to paint. I mix up an outline color with textile medium added, and I lightly outline a shape on the shirt.
Simple, organic shapes are good to start with if you're nervous about your skills. Mushrooms look nice and are supposed to be asymmetrical. Doing an image search for "mushroom" should help you pick out a shape you like.
Paint lightly. The lines don't have to be perfect, and they certainly don't need to be solid, especially at this point. One advantage of painting freehand is that you can vary the type of stroke, making some lighter, some darker, some thicker, some thinner. Play this up. Make it obvious that your shirt isn't stenciled. A painted t-shirt that mimics a rough sketch can be visually striking.
The outlining should also give you a feel of the fabric. T-shirts vary in weave and material; they'll take the paint in slightly different ways.
It doesn't hurt to practice drawing your design on paper if you feel inexperienced.
Step 3: Add some color (or not)
If you do opt for many colors, I recommend starting with the main color first. Follow with adding shadows then highlights. You might want to stretch the shirt a bit to make sure the color is getting into the grooves of the weave. Better penetration means better durability and stronger color. Yes, I said penetration. Stop giggling.
Using the paint undiluted (except with the textile medium) at this point will make the colors resistant to any "watercolor" washes you use in a later step.
Be careful when you're trying to get full coverage. If you add a little water to your color, it will be easier to press into the fibers of the shirt, but it's also more likely to bleed. It helps to touch your brush to the paper towel after loading it with paint and before painting the shirt. It blots any excess water and it shows you how runny/likely to bleed the paint is.
Step 4: Translucent brush strokes
Dilute your chosen color (already mixed with textile medium) with a lot of water. Touch the brush to the paper towel to blot excess water from it.
Start moving the brush BEFORE you touch it to the shirt. Very lightly skim the surface of the shirt with the bristles, moving them closer to the shirt as you lay faint color down. Usually I do this with a back and forth motion, as if I'm lightly coloring in an area with the side of a piece of chalk.
Do not press the bristles firmly against the shirt for this; the color will bleed.
Step 5: Color wash
This technique can make it look like you've used watercolor on your shirt. Be careful, though... paint bleeds a lot more on a t-shirt than it does on watercolor paper. It's really difficult to control the direction and amount of bleeding.
Dilute your chosen color (still important to have it mixed with textile medium) with lots of water. Make sure your other paint on the shirt is dry. Begin as you would with the translucent brush strokes, but before they dry, dip the brush in your water, and touch it firmly to your shirt. The water will spread and so will any wet acrylic paint. Keep dipping and touching your brush to wet any areas you want to have the effect.
This will take some experimenting, but it's fun.
If your shirt is soaked already and you brush a color across it, it will bleed some but still retain a darker color where the brush stroke was.
Step 6: Training wheels - using a partial stencil
Personally, I have issues with murloc anatomy. I'm more used to drawing humans.
This is where the partial stencil comes in. Once you have a sketch on paper that you're pleased with (create it any way you like, even if you have to print something out or trace it). Cut it out with a thin blade, hold it in place on the shirt, and paint light brush strokes (with diluted paint) over it from the inside outward.
Once you have a general, shadowy outline, you can sketch in the main lines and continue from there. I like the look of the shadowy outline, but you might prefer to trace your pattern some other way.
Step 7: Heat set and wear
I could also iron the stuff to heat set it, but that would require finding the iron... and I never bothered to get an ironing board.
I'm not into ironing.
I really hope you try painting a shirt at least once. Thanks for reading!