First, shake off any solid matter into a toilet. Then carefully fold the pad and place it in a sealed compartment until you are able to clean it fully.
When you are ready to launder it, place it in the washing machine and use cold water with standard laundry detergent. Do not use fabric softener because it will make the pad repel liquid instead of absorbing it. Do not use bleach because it can damage the absorbent fibers in the pad.
Dry the pad in your dryer on low. Note that you will probably have an extended dry time because the spin cycle won’t be able to get all the water out of the absorbent fabric.
You should always wash reusable underpads before using them the first time.