Can be done on the car but probably easier to take the springs off.
The add-a-leaf goes on the bottom of the spring pack on the front or between the main pack and the helper leaf on the rear ( depending on whether you have the helper ).
Simply take the bolt out of the middle of the spring, add the new leaf into the pack and put the new ( longer bolt back in ). Clamp the spring pack tight with a G-clamp and then tighten the centre pin.
Then put the spring back on the car! Simples.
If you do not have about 8 / 10 mm of thread left on your existing U-bolts you will need some longer ones for the front. On the rear I would be tempted to leave the helper off the bottom anyway which makes up the room on the std U-bolt for the add-a-leaf