Pull up the old pad and haul it out of the room. Allow the pad to tear wherever a staple holds it to the floor. It's not a big deal. Just rip up the padding as best you can with your hands.
Slide the end of a mini pry bar beneath a staple and pry it up out of the floor. If only one side pops out, grip this end with a pair of pliers and pull the staple out of the floor. Repeat this process until all staples are removed.
Pick up all under-pad fragments and toss them into a garbage bag along with the staples. Throw the old under-pad, fragments and staples away in a trash receptacle.
Slide the blade of a floor scrapper beneath the under-pad and rip it up. It doesn't matter if the pad tears. Scrape away as much of the pad as possible. If it comes off in pieces or strips, that's okay. Keep working until only the adhesive remains on the floor.
Apply carpet adhesive remover to the floor with a 9-inch roller equipped with a 1-inch nap roller cover. Screw an extension pole into the roller handle and dip the roller nap into a paint tray filled with adhesive remover. Roll the floor area farthest from the exit and work toward the exit until the entire floor has been coated.
Test the softness of the carpet adhesive after 20 minutes using the floor scraper. Slide the blade across the floor surface to see if the adhesive scrapes up. Scrape all removed carpet glue into a bucket and throw it away.