. First of all, you need to unmount the hotbed and turn it upside down.
. Place a strip of kapton tape on the hotbed contacts, since the termal sheet is covered in aluminium and you don't want to have some short-circuit.
. Cut the sheet in a square the size of the hotbed, minus about 5 mm per each side; you will need this gap to fix the insulation to the hotbed. Also cut a hole in the center where the thermistor is. (in one picture you will see kapton tape on the thermistor, after shooting it I removed it to avoid it giving false measurements caused by hot air trapped inside)
. Put some glue on the edge of the insulating sheet to give the tape a better grip.
. When glue is dry, place the sheet onto the bottom side of the hotbed and place the kapton tape all around the edges, fixing it to the hotbed.
. I put a small piece of tape in the center, to avoid the sheet loosening with time - again, not covering the thermistor.
. Reinstall the hotbed, connect the wires and do the leveling procedure again.