Firstly harvest them, cut the plant out by the root and discard any soiled leaves and all the yellow flowers. Early spring and late autumn are the best times for dandelions, in summer they get to bitter to enjoy. Once harvested, process the leaves by picking them away from the main stem and discarding any stems and strangely leaves.
The soak in some water for an hour then rise and shake the leaves to get rid of as much dirt as you can, you can be quite rough with them.
Dry the leaves and set in a bowl, to the bowl and a drizzle of lemon juice and equal parts good olive oil. mix into the leaves by hand.
Finally add some salt, pepper and some crushed up Thyme and mix in well. The taste in very strong, unusual and peppery, after the first few mouthfuls though you get used too it and its actually quite pleasant. I'd advise against selecting the larger leaves as they tend to get a bit bitter. I ate it with fried mushrooms and mash potatoes.