Go easy on the belts, DON'T soak them.
Pull each belt out fully whilst cleaning and don't retract them till they are bone dry.
Only need mild detergent or a very very weak apc mix, mist n wipe till your happy the craps gone then go over as many times as needed with a dry cloth to remove excess moisture,
Another tip for stoping mould coming back during the drying process is put a drop or two of zoflora in your apc mix which will stop the bacteria reforming while they are wet.
Quickest way to dry them(safely) is leave them fully out as I said above and run your engine for an hour or two(if possible) with heaters on.
Don't try and dry them out with a hair dryer.
Sorry if that sounds bossy but safety is a must when messing with seat belts

Hope this helps.