First, if your fabric is wrinkled, make sure to iron it first. Tape your fabric to a flat surface (table or floor) so it doesn't have any wrinkles or move around while you stencil.
When stenciling on fabric, spray adhesive is very helpful. Lightly mist your stencil with a spray adhesive, let it dry for a few seconds and place your stencil on your fabric surface, pressing it to achieve a tighter seal.
Use stencil brushes (if it is a smaller project) or a dense foam roller for a larger one. Add some textile medium (sold in craft stores) to your stencil acrylics and stir it in well. You can also get special fabric paint but frankly, good craft acrylics with a dollop of textile medium work just as well and cost less! Use very little paint on your brush and always off load any extra paint onto a folded paper towel. When using a stencil brush, apply the paint with a dabbing/swirling motion.
After all of your stenciling is complete, let the stenciled fabric dry completely (at least 24 hours). Now it's time to heat set the stenciling. Slowly iron the stenciled fabric by pressing the iron for about 20 seconds on a low setting through the piece of soft thin cotton cloth. Heat setting will cure the paint and will help to prevent it from being washed off in the laundry. It's always a good idea to first do a test with a sample of your fabric to ensure good results. Do your stenciling, heat set it and wash the swatch to make sure the whole technique works on your fabric. If the paint washes away you may need more heat-setting time, but it usually holds up just fine. Some fine fabrics will not tolerate heat setting and it's always more difficult to stencil on sheer fabrics.
If too much paint is used during stenciling, it may leak through on the other side of
the fabric, which may add stiffness to the fabric. If your fabric is thin, place some paper (or newspapers) under it to protect your working surface (table or floor) from possible paint seepage.
We've stenciled on silk, cotton, linen and even sheer fabrics with great results but's always better if your fabric doesn't have too much texture.