Take the car for a short spin to warm the oil. Jack it up (remember it has to be level so you'll need ramps one end/axle stand the other or something similar).
Before removing drain plug, make sure that the filler plug is loose. This is because you may have difficulty loosening the filler and you don't want to end up with an empty gearbox and no way of re filling it 
Here's the location of the drain plug:
Drain the old oil (24mm socket required), clean the muck from the magnetic drain plug, then take out the refill/level plug on the side of the box (16mm socket will suffice).
Replace the drain plug (use a new sealing washer, it's identical to the oil drain plug one).
Try to obtain a flexi plastic bottle like this one:
Manouver the oil bottle up alongside the gearbox, above the filler hole, like this:
Keep filling, until the oil overflows from the filler provided that your car is level to the ground this will ensure the correct oil level in the gearbox.
Once that's finished put the plug back in,and you're done!
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Disclaimer
Any work you do to your car is entirely your own responsibility. If in doubt, you should check technical advice with an independent , qualified person who has seen your car. The MX-5 Owners Club, its officers and forum contributors accept no reponsibility for any damage caused to your person or property as a result of you following or not following the advice offered on this forum.