Chop up the swede in quite small pieces and cook in boiling salted water until tender to point of knife.
Then, drain really well. This is the trick, soggy swede doesn't mash well. Drain in a colander, then tip back into the pan on medium heat on the hob and shake it about to get rid of excess moisture.
Then, mash in the normal way, with more butter than you would believe possible, I use loads, a good eighth of a block of butter, more if I am cooking more swede. Then loads and loads of black pepper. Then beat with a woooden spoon to make fluffy.