Use tape to get straight lines!! This is how I get all my court lines in my tennis paintings and any straight edge I need!
Tips:
Be mindful of watering down your paint too much, if your paint is too wet/runny it will seep Below the tape line.
Also, I remove the tape right after, when the paint is still wet, to avoid a weird edge.
Clean up any jagged edges or blurred edges with extra paint and a fine brush after the initial paint line you made with tape has dried (I have tried to fix or wipe paint wet and it usually doesn’t work).