Cut up all the quarters like this and your celeriac is ready.
Remarks
The fresher the celeriac, the harder it is to cut, so use a really large knife to make this easier.
Once the cut surface is exposed to the air it will start to oxidise and turn brown. If you are not going to use it immediately, wrap the pieces in plastic film and refrigerate, or put them in cold water with the juice of a lemon or, better still, a pinch of vitamin C powder.