well...it depends on the material. Leather collars must be hand washed, chain collars don't get washed, The collar I had was a strange basic material that I put in the washer and now its barbie sized x.x, and any other collar's I am missing, its probably better to just hand wash them.
So like get a wash rag and wash it with flowing water. You would need like a hose or a sink. Rain usually doesn't effect collars so if you like to run out in the rain like me, jus dont wear a leather collar and your good to go.