How do i clean baseball caps without making a mess?

Hand wash in a bowl, not the machine. Warm water and a small amount of laundry detergent, soak 15-20 minutes, then a soft toothbrush on the sweatband, that's where all the grime is. Rinse, press the water out with a towel (don't wring it), and dry it over a rolled-up towel so it keeps its shape. Keep it away from the radiator. If it's an old cap, check the brim first. Some older ones have cardboard in there and soaking ruins it, so spot clean those.