To get realistic branches, you’ll want to thin your paint out with water first. Then use a liner brush. A long liner brush is ideal because it holds more paint and you can pull longer lines with it. Apply less pressure and be free with your hand as you get toward the end of the branches. Let them bend and take small angles so they look more natural. You pretty much want to use the brush like it’s a pencil or pen. The more pressure you apply the thicker the line will be, the less pressure the thinner the line.