Stencils with a self-adhesive backing are easy to use, especially with unstable or flimsy textiles like T-shirt knits. If your stencil does not have the adhesive backing, the Stencil Source suggests using a spray adhesive on the back of the stencil. Allow it to dry to just tacky before placing on the fabric. Masking tape holds the stencil in place on most woven fabrics.
Liquid paints come in a greater variety of colors and are less expensive than cremes or paint crayons, but the nature of the textile will also be a factor in choosing paints. Liquids provide a uniform area of color and work well with smoothly woven fabrics like cotton, polyester, silk, and lots of specialty fabrics. Cremes and "dry" paint crayons work best with textured fabrics such as linen, and make it much easier to create shading of your design. If you already have a supply of acrylic paints on hand, invest in one bottle of textile medium, mix it in equal parts with your acrylics and you have a ready-to-use textile paint.
Sponges or brushes work equally well. Sponges should be dense foam (triangular make-up sponges are excellent) and stiff-bristled stencil brushes are best. Since you should never paint with a wet brush or sponge, you'll need one for each color.
Dry cleaning chemicals are not friendly to textile paints so the fabric should be washable. Pre-wash the fabric, including practice swatches, according to manufacturer instructions (no fabric softener); dry and iron smooth.
Practicing on a sample of textile is a good idea before starting the main project. Place a piece of cardboard under the area to be stenciled. Position the stencil and secure with masking tape if it is not self-adhesive.
It takes a very small amount of paint to be effective and you can always add more. Blot the lightly paint-loaded applicator on a paper towel until very little paint is left. The fabric and desired effect will determine whether you "swirl" the paint in a circular motion or "stipple" in an up-and-down dabbing motion. The fabric may move under a swirl, so you may prefer to stipple. You might need several light applications of paint.
Most textile paints need to "cure" for at least 48 hours or as long as two weeks. The instructions may call for you to heat-set the paint, either in a clothes dryer or by covering the design with a cloth and ironing. Follow the manufacturer's instructions for curing and/or heat and future washing instructions.