My son recently took his tractor driving test. He is 16 and I looked everywhere for some information on the test and/or to try and find an instructor...but unfortunately couldn't find anything.
You will need your own tractor (or borrow one) to take the test. At the test, they go through all the things you would be tested on for a practical car driving test. The examiner does the highway code questions and asks about different things to do with the tractor etc. Then tells you where to drive and he watches and follows where necessary. Hill starts, three point turns, reversing round corner etc.
The ironic part of all this is that you aren't allowed out on the road to practice before the test, the first time you can drive on the road is to get to the test.
If you already have a car driving licence then I would think that taking the tractor test wouldn't be too much different.
Hope this helps.
If you want to know more about the test send me a pm and I'll get Rob (my son) to write back.