Before you start cutting, make sure there are no pipes or screws under your cut lines. (If there are, you can always remove them temporarily.) Measure the thickness of your bench and set your circular saw blade to cut 10mm deeper than your bench. Now cut along your lines, stopping just before you reach the corners. Then use your jigsaw to cut out the corners. Remove the cut out and your sink is ready to install. You can make this job a little easier by drilling a hole on the inside of your cutline first. Then use than hole as a starting point for cutting out with your jigsaw, and finish off with the circular saw.