Make sure you check the washing instructions on your product as each brand is different.
Most brands recommend that you rinse the pants with cold water and then throw them in a cold black wash, preferably in a laundry bag (as is recommended with any high quality underwear). If the pads are heavily saturated, consider soaking them for an hour before you wash them. You can then hang-dry the undies inside out, bearing in mind that they will take longer to dry than your normal pants because of the absorbent layers.
Fabric softeners and tumble drying are not advised as they may decrease the performance and absorbency of the pants.