First, cut off the tops, then continue to slice turnip into 1/2" slices.
Then, to avoid bitterness, pare past the dark line separating the rind from the white center. (turnips are a little "twangy" anyway- this keeps them from being overpowering!)
Cut into chunks, then microwave with a little water & salt, or steam them, or boil them (the least nutritious alternative) until tender.
I mash them up with a little cream, butter and salt & pepper to taste, using a potato masher if you like a little texture, or blender or mixer if you prefer a smoother mash.