First buy a new expansion tank cap for $5.
Then, with car off, open the heater valve all the way. Remove the thermostat and leave the housing off. Open the expansion tank cap. With the car off, add coolant to the thermostat opening until full. Then add some more into the expansion tank until it runs out of the theromostat opening.

Reinstall the thermostat and housing cover. Start the car and let run for a few minutes, add some more coolant, if necessary. Put the cap on the expanion tank, and let the car heat up. If it gets too hot, run the heater full blast to keep it from overheating.

If it is still overheating, you must have a blockage or a blown headgasket.

Good Luck,

Charles