How do I make a homemade stencil brush?

How small of a brush size are you seeking? Please tell us a little more about what your project is, so that we can try to answer your question.

You could use make up sponges

This video helps me to create mine so maybe this could help you: https://www.youtube.com/watch?v=T_KjKmwy9TA

kitchen sponge, Q-tip, cotton balls...all may work as well!

I have tried a kitchen sponge but found the holes are much larger and I didn't like the outcome. My next attempt was to slightly dampen a Q-tip, smooth the fibers with clean fingers ( no lotion), and allow to THOROUGHLY dry before use. I haven't paid for a sponge brush since! Note: All cotton top swabs are not created equal, the extra pennies for name brand are well worth it.

In the past I've used one of those little cheap bristle brushes. I wrap a rubber band around the bristles and then trim the bristles. Easy to have a brush for each color for a project. Also, if you apply too much color you can put the stencil back in place and correct the color using the wall paint...just lightly tap until you are pleased with the result.

I have not done stencilling myself, but I am wondering if empty bingo dauber bottle tips might work ?

I saw these tips and holder on Amazon.

Use make up sponges.

You can use a small paint brush just wrap tape around the bristles first.