If they're breathables, DO NOT use any form of washing powder or detergent or you'll screw up the breathable properties. Most manufacturers recommend
Nic-Wax Tech Wash (get it from Amazon or the like).
Just stick it in the washing machine with your trousers. It is important to run a complete cycle of the machine with no detergent before using this stuff - if there's any soap in it, it won't work.
You can also get Nic-Wax TX Direct which you bung in the washing machine after the Tech Wash, this will re-apply the waterproof 'run-off' outer coating to yer breeks.
It's great stuff, I've been using it for years on breathable motorbike bike clothing as well as fishing kit