According to "Fabric Painting Basics," acrylic dries stiffer than other types of paint. This makes items less comfortable to wear. Paint items like hats and bags with acrylic paint to avoid this problem. Save T-shirts and other clothing items for when you have fabric paint. Acrylic paint does not come off in the washing machine, similar to fabric paint. Acrylic paints also generally cost less than fabric specific paints.
Place the fabric item in the washing machine. Wash it as usual, but don't use any fabric softener or bleach. Dry it without dryer sheets.
Place a piece of cardboard in between fabric items with layers, like T-shirts or trousers. Thin cardboard, like from food packages, works well.
Put small drops of paint onto a flat surface, like a paint palette or paper plate. Dip your painting tools into the paint and decorate the item. If you wish to use a stencil, place the stencil on the item. Tear off small two-inch pieces of masking tape. Tape the stencil to the fabric item, and then the fabric item to the flat surface underneath to keep both from moving. Remove the tape and stencil when you are done painting. When painting, do not use thick coats. Thicker coats of paint are more likely to crack or flake apart.
Clean out the brushes and painting surface immediately with water and soap.
Let the item dry for at least 24 hours; if the room is humid, it may take longer to dry. Remove the cardboard after it dries.