There should be a recipe for celeriac and leek mash on the vegetarian recipe thread.
But if not, it's very simple. Peel and cut the celeriac into chunks, then boil in salted water till tender (20-25 minutes, depending on the size of the chunks).
Top and tail 1-2 leeks (depending on size) and peel off the outer layer - wash carefully as sometimes they can be gritty. Slice into rings about 3/4 cm wide. Add these to the celeriac and cook for another couple of minutes, until the leeks are tender.
Drain this into a colander or sieve, and leave to drain and steam dry. Then tip them back into the pan you cooked them in, and put over a low heat.
Leeks retain a lot of water, so it's a good idea to do this, as you don't want the mash to go to water.
When it's quite dry, add a very generous lump of butter and mash - either with a masher or with a stick blender. Taste, and then season to your liking with salt and freshly ground black pepper.