A Miata coolant flush is simple, but the air-bleed is the step that makes or breaks it. Trap air in this engine and it overheats even with the system full. Take your time at the bleeder.
Drain. Cool engine only. Open the radiator drain or pull the lower hose and catch the coolant. For a fuller flush, run water through until it comes out clear, then let it drain completely so you are not diluting the new mix.
Refill mix. Use a 50/50 of proper ethylene-glycol coolant. The NA/NB take roughly 6 quarts total system capacity; NC/ND differ, so fill to the marks and top after bleeding.
Find the bleeder. There is a bleed screw near the thermostat housing/upper hose area. Open it before you fill so air can escape as coolant rises.
Slow fill. Pour into the radiator neck slowly. When coolant dribbles from the open bleeder with no bubbles, close the bleeder. Top the radiator to the neck and set the cap loosely at first.
Burp it. Start the engine with the cap off, heater set to HOT (this fills the heater core). As it warms, the thermostat opens and the level drops as air purges; keep topping the neck. Squeeze the upper hose to help bubbles move. When the fans cycle and the level is stable with the upper hose hot, cap it.
Confirm. Watch the temperature gauge through a full warm-up and a fan cycle. A gauge that climbs past normal while the upper hose stays cool means the thermostat is shut and you still have trapped air or a stuck stat. Let it cool, re-bleed, and top the overflow bottle to the FULL line.
Check next day cold and top the reservoir once more as the system settles.