Check all of cabinets are level and securely fixed back to the wall and all of the plumbing and electrical work is adequately advanced. Under counter appliances should be removed and preferably, cupboard doors not fitted.
I always work clockwise, when cutting worktops with a bull nose, it is much easier to work clockwise, you can fit a worktop and it's easy to mark the angle and cut without spelching, but more on that later.
So measure the first worktop, check the walls for square by putting a long spirit level or plinth against the length of the wall and square off using a large roofing square, your jig or a cupboard door. It's normal for the wall to be between 5 and 10mm out just in the 600mm of worktop. I like to sketch a plan of the worktop showing the dimension at the back and the dimension of how much out of square the worktop is. 
I always take 8mm off the dimension, this gives a 4mm gap reach end which can be sealed with silicone, tiled over etc. but gives you the required play to get the worktop in. To cut this worktop a circular saw is adequate, make sure the worktop is supported at each end and both sides of the cut. I use 4 pieces of 600mm 4" square timber for this. You may be able to buzz straight through the worktop in one pass but it is probably more sensible to do 4 passes at 10mm (for a 40mm worktop).
http://www.asandow.co.uk/Sheffield-Joinery-and-Carpentry/Worktop-Mitres.html
To cut without spelching, make sure the worktop is upside down, and you start on the front edge. If the worktops have a preformed bull nose, you will need to cut the female mitre following the instructions for your particular jig. You can cut in 10mm passes with some woods but some you may need to cut as small as 3mm passes. You will know if your router is struggling because they slide through like hot knife through butter when your cutting the correct depth. They scream, judder and you'll see plenty of little electrical sparks near the motor. As I slide the router along the jig, I push down with my left hand to keep a constant weight above the worktop and I push to the right with my right hand to keep the guide bush tight to the jig. If you let the right hand of the router go down you will cut into the worktop where you don't want to a do not get a perfectly straight cut. You can then mark where the worktop bolts will go (between the various strengthening bars at the top of the cabinet). and cut them out with the jig.
http://www.asandow.co.uk/Worktops.html
Place the worktop in place and proceed with the second worktop. I cut the second worktop about 50mm too long, the place it on top of the first worktop on the left hand side. Check if the right hand side needs cutting to suit an un square wall or to line up with the end of a cupboard or appliance if this is the last worktop to be installed, cut this end first the re-place the worktop withe the right hand side where it will be on completion and the worktop tight to the back wall, now putt a pencil mark under the left hand side against the first worktop. Mark this scribe, mark the jointing bolt homes then put the worktop upside down in your cutting area. Line the jig up to the line, bear in mind that a 30mm bush and a 13.5mm straight felted cutter will mean you want the jig 8mm from the line. Clamp the jig on with the pin in the joke for that size of worktop on the nose and proceed to cut the worktop in approximately 10mm passes as before. Cut your worktop bolt holes using a depth stop guide on your router. Turn the worktop the correct way up and proceed to cut the female mitre as before if required.
If the Worktops do not have a preformed bull nose, and you intend on cutting one, I would recommend the following. Set one of the depth stops on the router and cut any part of the worktop which will be impossible to get at later; ie tight to the wall, and then when everything is installed buzz around with the router, if there is an open cutout for a Belfast sink, round this off before cutting drainer flutes as the bearing guide may bump into the flutes.
Safety.
It's with noting that Worktops are very heavy, a 4m 40mm worktop is probably too heavy for two people to lift. It goes without saying that you'll have suitable PPE and RPE on, eye protection, ear protection and dust mask are a minimum. Also, be very careful not to cut your own electric cables!
Cutting for hobs and sinks
For the most simple hobs and sinks, as a rule of thumb, you can turn the appliance upside down, draw around them, come in 10mm and jigsaw them out. Use a down cutting jigsaw blade to avoid spelching. For some gas hobs, use the dimensions in the instructions. For under counter sinks or open Belfast sinks use a jig.
If you need any further assistance give us an email at info@asandow.co.uk or call 07921317375
