Drain radiator, remove airbox, remove throttle body, undo hoses, undo thermostat, get fluid everywhere, new thermostat in, replace throttle body and hoses, replace airbox and refill coolant.
It would probably take me between 1 and 2 hours including swearing breaks and tool hunts. A calm mechanic would probably take less time.
Direct from Mazda the thermostats are spendy, get a ford one from Opieoils or similar for £20ish
Do note there are two different sizes of thermostat for Mk.3s, you’ll need the one with the right length housing. (I stumbled on that when seeking a new stat for Mrs Y’s Mazda 6 which has the same basic engine.)
You can buy the long hose outlet stat and trim the hose by around 30mm, that’s what I fitted to mine. Mine took the shorter version.
Mahle TI 202 82 is the model, available on eBay.
Opie oils used to do them for around £13, appear not available now. As said eBay have them for not much over £20 delivered.
Of course Mx5parts have both sizes to suit whichever model short/long outlets at a premium price.