log on to Thai UK website. Pretty straightforward got mine in 6 working days. 180 days every time you arrive in Thailand, last for 5 years. You can extend the 180 days for another 180 days, but never tried myself.
what does it save? You get 60 days both ways and have to extend for 30 days. Done it numerous times in UK online, 90 days gives you 60 and still have to extend for 30 days in Thailand. Less than 40 minutes in immigration, not a big issue.