To install a carpet in a car, remove everything fastened to or through the old carpet, remove the old carpet, measure and cut the new carpet, install the replacement carpet, and bolt all of the items that were removed back into place. Choose a carpet that is flexible enough to conform easily to the humps and bumps of a car floorboard. The items that need to be removed vary depending on the vehicle's make and model.
Park the car on a flat surface that provides easy access and room to open all of the doors. Remove the sill plates along the bottom of each door. Remove the bolts for the seatbelt anchors as well as the bolts holding the seats in position. Disconnect and remove any electronics attached to the carpeting such as seating position controllers. Unscrew the mounting screws, and remove the center console if there is one. Remove any floor mounted shifting levers and associated mounting plates.
Remove the old carpet, and lay it on top of the new carpet to use as a pattern. Trim the new carpet to fit, leaving a little extra all the way around that can be trimmed once it is in place. Clean the vehicle floorboard, and treat or repair any rust spots. Screw the seat mounting bolts in before inserting the new carpet. Place the new carpet, and locate the seat mount bolt heads by feel. Use a sharp carpet or utility knife to cut an x-shape directly over them. Remove the bolts, position the seats over the holes, and fasten them securely.
Reattach everything else in a similar manner. Trim any excess carpeting at the door openings, and reattach the sill plates.