The easiest to do are the ones with plastic Lionel couplers. With a Dremel cut-off wheel, remove the coupler and leave as much of the plastic behind the head of the coupler as you can. You will notice a faint line of flashing running halfway along the side the remaining piece of plastic that joins the coupler to the truck. You will want to cut the top half off running about 3/4" back toward the wheels. You will mount the Kadee coupler pocket on this "shelf" that you have created. I used 2-56 machine screws. Unfortunately, not all manufacturers use plastic couplers. Some use metal and removing the coupler from that metal piece takes more time and heats up the metal quickly. Ouch! Cutting down and across to make the shelf for the coupler pocket to sit on takes more time. You will need to wear safety goggles! As you cut to make the shelf for the pocket, it's better to cut less than more as you trial and error your first attempts. I've attached pictures. The ink shows what remains on which the Kadee coupler pocket rests. Hope this helps.
Terry