In order to install carpet yourself, begin by removing old carpet, then put the underpad down, climatize the new carpet, make measurements, and cut the new carpet. Set the seams of the carpet, and attach it.
Begin by removing the door and any molding, and cut the existing carpet into strips about 2 feet wide. Pull the carpet and underlay off the tackless strips, and then remove the strips themselves. Install the new tackless strips around the edges of the room. Lay down the underpad in adjacent strips that overlap the tackless strips.
Allow the carpet to rest in your home for up to 24 hours to acclimate it to the climate of your house. Lay the carpet in a strip that overlaps the edge of the floor, and then trim to fit. When installing the next strip of carpet, allow it to overlap with the first, then cut through both strips to ensure matching edges, and use a seaming tape and iron to join the pieces of carpet. Use a knee kicker to attach the carpet to one edge of the room, then a carpet stretcher to situate the carpet properly over the tackless strips. Perform any remaining trimming necessary.