Coolant capacity is 6 litres for Mk1 and MK2/2.5.

Tools needed:
Screwdriver for radiator plug
Pliers to release hose clips
Ramps or axle stands
Funnel
Jug
Bowl large enough for 6+ litres
Pre mix a 50/50 batch of antifreeze and distilled/de mineralized water.
Obtain new radiator cap

Drain the water from the radiator into a suitable container, by undoing the large plastic plug in the bottom of the radiator. Be careful with this as the plastic is quite soft, so make sure you use a reasonably large screwdriver.

It can be worthwhile loosening the bottom radiator hose to make sure that all the water is out of the block, remember to tighten it and the drain plug up before re filling.

It is worth the effort to back flush the radiator, this needs both hoses to be removed. Water flows from top to bottom of the raiator, so pop a garden hose into the top hose opening on the radiator.Use gentle pressure from the hosepipe.

Put the car up onto ramps or axle stands, and make sure all hoses are re fitted and that the drain plug is in.

Pour the pre mixed fluid into the top of the radiator until it is full. Gently squeeze the top hose, this promotes a movement of water around the block and helps to move any air from the system.

Start the engine, with the car still on the stands/ramps and let it tick over for a while until the thermostat opens [you can tell when this happens as the water starts to move in the radiator filler hole].

Now once again gently squeeze the top hose, you will see bubbles coming out of the radiator filler. Keep doing this until you think you have all the air out.
Top off the radiator right to the top and replace the pressure cap [It's quite a good idea to replace the cap at the same time as you replace the coolant, pretty cheap and always buy the genuine article from Mazda].

Lower the car to the ground and have a short drive, keeping an eye on the temp. gauge. Back home check the level once more after the engine has cooled down, top off if necessary and jobs done.

It makes no difference on an MX5 if the heater controls are in the heat position or not. Hot air is controlled by a flap and not a water valve.

Dispose of the old coolant responsibly, i.e. not down a drain!