with my waterproof rugs i usually keep them on the horse and tie the horse up
scrub them with a water and brush, or use the hose, [its a good way to get the pony over being near water haha , my pony’s just don’t give a cra”p anymore 
and when i was cottons i do up the leg straps /belly straps and chuck em in the washing machine you?
i do it maybe, every two months when she decided to roll in mud of have a few to many birds use her as a perch >.<