Stenciling can be done over bare wood, a previous coat of stain or a finish applied earlier. For unfinished wood, however, a coat of water-based finish should first be applied to the entire piece, not just the part to be stenciled. This will seal the wood and make it possible to wipe off any spills, runs or mistakes. Stain spilled on bare wood, would have to be sanded off.
Someone who is artistic, may want to make a stencil from acetate. If not, use a ready-made stencil and spray one side lightly with stencil adhesive. Then press it firmly onto the space to be stenciled.
Note: Never stencil antique furniture: it will reduce the value.
Safety alert: Be sure to wear plastic gloves when working with stain.
For more control, instead of a stencil brush, use a small round sponge to apply the stain. Dip the sponge into the stain and then dab the excess off on a cloth or paper towel. The sponge should be almost dry.
Dab the sponge over the stencil. Light pressure is all that is needed. The idea is to deposit the stain on the wood, not to force it. For an unusual effect, do some twists and turns with the sponge, or add extra stain to parts of the stencil to create a mottled, aged effect.
Let the stain dry for at least half an hour. An additional color can be applied on top of the first color of stain.
Peel the stencil from the chair. Give the entire chair a coat of water-based finish to protect the wood and the stenciled area.
If there's still some adhesive on the chair when the stencil is pulled off, gently scrape off the glue with a fingernail.
Stains can be mixed together so long as water-based is with water-based and oil with oil. Keep a record of the mixture, so it can be duplicated if desired.
To use multiple colors on one stencil, wait around 15 to 20 minutes. One of the advantages of water-based stains is that they dry very quickly, especially when it's warm and dry in the work area. If it's not, the process can be sped up with a hair-dryer on a low setting.
If finishes that come in aerosol cans tend to plug up the sprayer after the initial use, when spraying is done, turn the can upside down and press the tip until all the finish has been expelled and nothing is coming out but propellant. That step will blow the finish out, so it won't dry in the nozzle.