As simple as it can be. Peel and chop the turnips (or swedes if that's what you call them) and put them in a big pan with boiling water. Add salt and pepper and simmer for up to 2 hours. Remember to replace the water as often as you need to. The neeps will be ready anytime after around 20 - 40 minutes, depending on your cooker and the size of chunks you cut your vegetables into, as well as your preference. We like long boiled darker mashed neeps here..
When cooked, the neeps will be easy to put a knife into and simply mash them with a little butter until the lumps are all out. Some people add a little milk, but I prefer mine without.
If you want to keep them hot while you are making a full meal ready, just put a knob of butter on the top, cover with tin foil and put in the oven on a low heat.