Bathtub works. And, imho, a nice warm shower is even better, as you can better clean the whole mat easier with handsoap, and cleanse it with clean water properly.
Can you iron these mats without causing damage? Mine will not lay 100% flat on my desk the ends are curled up. I have tried to weigh it down with books but it does not seem to be working. It is very sad because the mat looks great and is very high quality just want it to lie flat.