Most countertops are roughly the same depth (this is to accomodate the cabinets makers, appliance manufacturers, etc) so if you were installing a square or rectangular benchtop this would be a simple case of buying a prefab one from the store and cutting it to length. In my case I had a corner to deal with and decided to go with a mitre join.
I started by measuring the existing countertop. Be sure to add a small overhang so that you can cover the existing benchtop. I extended mine by roughly 30cm.