Well, you should mark it out first. Tape a big sheet of paper on the bonnet, that well covers the area likely to be cut.
Then pop the lid and measure the location of the aircleaner centre - how many mm from the side of the front guard , how many mm from the back edge of the engine bay. Close the bonnet and transfer these dimensions to get the centre of the cutout. Then it's easy to mark out the rest.
Shade it or colour the paper appropriately, stand back and have a gander. If it looks ordinary, better to know now than after cutting. Assuming you're happy, then open the bonnet again and look at what has to be cut underneath. Typically, it runs right through a diagonal brace...
If you have the bonnet assist springs, take them off. Weakening the bonnet by cutting will make it way flexy and the thing will kink near the hinge mounts, from the resistance of the springs to closing. You need to use a prop from another vehicle - easy to fit.
Dores this help? I am planning a power bulge in mine to clear the triple carbs. Hope to spring for louvres, too.
Cheers, Adam.