Easy peasy.  Reusable pads have wings that fasten around your undies with a snap. They are swapped out just the same as you would a disposable pad, but instead of throwing them into an icky garbage can where they'd go to the landfill and sit there for hundreds of years, instead you place them into a container or bag to await washing at the end of the cycle, or whenever you would like to wash them. And, when you do so, you can give yourself a big ol' congrats for being so planet-friendly!