Step-by-step: Three steps to a custom Sticker brush
1 Paint the marks you want to make up the Sticker brush into a grid formation on one layer. Pure red marks will afford you more colour control later. Paint marks left to right, or right to left, depending on which way you want the sticker brush to flow. Export the layer as a PNG to keep transparency information for the image.
2 Click the Stickers tab to open a palette of all the Sticker sheets available. Click New to open the Load pane. Click Colour and open your saved PNG file. Repeat for a second layer. Click Texture if you wish to load the same file for texture. Pick Shadow (or not). Set rows and columns appropriate for your sheet. Save with a suitable name
3 Close all the panels and select the Sticker Brush. Click Settings>Spray Variation. Each grid square is a control, like Pen Pressure/Scale, which you can drag from +100 to -100. Hue/ Tracing H 100 per cent gives more colour control, if you used pure red in step 1. I use Sheet sequence 100 per cent random; see what works for you.