If the kitchen is fitted with three worktops i.e. one at the end and the other two running at right angles to it, you will need to install the end one first.
Measure the distance from the wall to the edge of the cabinets and cut the worktop to the required length, allowing for any defects in the walls or awkward angles. If in doubt cut a template of any odd shapes. When all the worktops are cut, seal all worktop edges with PVA then fit the joining strips to the worktops using silicone sealant to seal the ends and edges of the worktops. Position the worktops on the base units and check that they all fit.
Fix the worktops down using suitably sized woodscrews and check the joints to make certain they are level and square. You will need to cut out areas for hobs and sinks etc. before finally fixing the worktop.
Worktops can also be jointed with mitre joints and connecting bolts.
Fittings worktops (and especially whole kitchens) is best carried out by a experienced installer as it requires special tools and a relatively high level of skill. Always seek advice before starting any DIY project.
Tools that may be required for the job: Saw, Sander.