Really stupid coolant question...

add the coolant through the resivoir. just keep adding it. make sure that the front end is jacked up and that will help get all of the air bubbles out.

i've always filled it up at the thermostat housing and then the radiator after that, but of course this is on older cars. havent worked on a mustang yet...

fill it like suggested with the front end raised on jackstands. fill it then start the motor and fill as needed.

After you fill and run the car (warmed up), let it cool completely...massage the upper and lower hose with the reservoir cap on and off (front end elevated). Top off reservoir with fluid. This will help fill the block completely and remove air. Do this a couple of times to be sure.

what i usually do is fill the overflow bottle. and while the engine warms up i bring the engine rpms up too 2500 or so. this will break up the air pockets in the heads and heater core.

DO not just fill and go. it's going too to take a little while of bleeding too get it all out. and without one of cooling system fill machines the only way too get the air out is the way i mentioned. be ready with more coolant because as the air pockets are pushed out the coolant level will drop.

have the heat on high blowing out the vent when you have good heat at idle recheck the level and put the cap on. recheck again after you drive it in case of more air int he system.