Get the dimensions of the cutouts from the installation instructions. Usually there is a tolerance, e.g. 480mm +5 -0.
Using a square mark out the cutout on the worktops - draw with a pencil so that it can be rubbed off if needs be. Draw the maximum and minimum dimensions - you can then aim to cut between them. Check the diagonals are the same length, if not, then your cutout will not be square.
Check underneath the proposed cut lines - there may be supporting brackets which could play hell with your jig-saw.
Finally, re-check you dimensions a couple of times, so you don't end up scrapping the worktop. Drill a small hole at two diagonally opposite corners to insert the jigsaw blade - the hole should be just tangential to the cut lines.
Make the cuts slowly with a jigsaw, you will generate a fair bit of dust so be prepared for a mess. You might want to wear ear-plugs, eye protection and a dust mask while you are cutting.
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