1) The first step is to determine which style of door casing you have, then match it up to the "inside profile" of the Casing Kickers on the product page.
2) Determine if your doorway is a "cased opening" (doorway with no door or doorstop). i.e, a doorway going from a kitchen to a dining room. Or a doorway with a swinging door. i.e, a bedroom, closet, or bathroom door.
Kickers for cased openings are routered across the entire top edge.
Kickers for swinging doors have "SD" in the item number and are left square in order to match up with the doorstop.
3) Casing Kickers come with a heavy coat of primer. I suggest painting with semi-gloss (or whatever color you choose) before installing. This way you have a nice clean edge resting on the floor.
4) Take the longer of the two interlocking pieces and place against the door casing as in the picture below.
Note: Door trim varies from home to home so you may run into an area where you need to trim, cut, file, or saw part of the "Kicker" for a good fit. Casing Kickers are made from wood just for this reason. Mark the area of interference with a pencil and trim, cut, file, or saw at that location as needed, then caulk into place. The caulk acts as the adhesive so they do not have to hook the backside of the door casing to hold in place.
5) Take the smaller of the two pieces and place into position to interlock with the longer piece and then press down evenly.
6) Now just caulk around both pieces and then remove the excess caulk by gently running your fingertip across the top of the Casing Kicker. The lighter the pressure the better the results will be. You can also use caulk to fill in the interlocking section of the two pieces.
7) For doorways with swinging doors you should first make sure you have 5/8" of clearance between the bottom of the door and the floor. Take your Casing Kicker and lay it on the floor against the doorstop as in the pictures below. Take a pencil and make a mark across the doorstop for a reference where to cut (I use my handsaw to make the mark). With any type of handsaw cut the doorstop only. Try not to cut into the board that the doorstop is attached to. However if you do don't worry, the kicker will cover it up. After you make your cut you can tap out the cut section with a flat-head screwdriver and a hammer. Then install the same way you would for a cased opening.