My daughter is a painter and decorator.
I knitted a plain cover and made a chart for a paint brush with red paint on it for the front.
As usual you need to work a test square to decide how many stitches and rows you need to do for the wool, tension and size you want the cover to come out. You can either knit a separate front and back, or do as I did and knit it all in one, then there are only two seams to sew up.
Once you know how many stitches and rows you will be able to work your chart over you can use squared paper to make your chart.
What can you make a motif of?