Lay the wood you will be painting on a flat surface and dry it off as much as possible with clean cloths. Press the cloths firmly into the wood across the entire piece. Flip it over and repeat the process for the other side and all the remaining pieces of wood you will be painting.
Clean off the surface of the wood. Use a broom to brush off all dirt and debris that will get in the way when you are painting. Repeat the process for the other side of the wood. Lay a tarp or piece of plastic between the wood you are painting and the surface it is sitting on.
Open up a can of your water-based paint. Mix it up with a paint stir stick, and dip a paintbrush into the can. Spread out the water-based paint over the surface of the wood. Start on one end and work to the other with smooth, even strokes.
Dip the brush back in the paint can, when necessary, and continue until the first coat is complete on the top side. Let it dry for at least one hour, and then add a second coat of the same color. Allow the second coat one hour to dry and flip the wood over.
Paint the other side of the wood with two coats just like the first was painted, and repeat the same process for all the other pieces of wood that you need to paint.