Not high tech, not fine detail, but if you just want something to splosh about with and don't want to have to clean your brush later, or you don't have access to your brushes.
A piece of scrap paper. I find I can make 3 brushes from 1 page of A4 (roughly letter size).
A tiny snippet of paper 5cm by 3cm ish, very ish.
Optional, something to roll around.
1. Take your piece of paper and cut it to the length you think you want your brush to be, the wider the piece of paper, the fatter the brush.
2. Along the long edge, start cutting little strips, so you end up with a fringe. The longer your strips are, the more re-uses your brush will have.
3. Roll up your fringed piece of paper and secure with tape.
4. Take your snippet of paper and roll around the brush and secure (to itself only) with tape, so it slides up and down the brush.
5. Remove the thing (if any) that you rolled around.
6. Slide the snippet towards the brush end until you have just enough fringed bits to use as a brush.
8. Cut off the bit that you used and slide the snippet back down the brush til your fringed bits are long enough to use again.
Necessity is the mother of invention!