You pay monthly. You either sign up for a year or two and get a free dongle or you buy a dongle and commit yourselves to just one month at a time. The dongle just plugs into a USB port (I use a cable so it doesn't get knocked) and it automatically installs the software and a screen pops up asking if you want to connect. Mine seems to work only if you plug the dongle in after the computer has had chance to fully start up.
I believe that it is possible to manage without a dongle (with a phone, cable and £15(?) per month sim card) but I haven't investigated this. I went for a 2 year contract with Vodafone and a free dongle. It's not as fast or as reliable as my last fixed line connection but it is perfect for the way I work - I just carry the dongle rather than a laptop with me.
You should see which mobiles have a good signal at the main addresses where internet will be used and you should order online or by phone. If you do this and then find that the reception isn't good enough at one location, you can cancel within 7 days under the distance selling regulations.
The last time I looked, Three had the best deals but also the worst coverage.