If you look at the underside of these benchtops they will have a set of 3 holes bored into them about 10mm from the edge of the join.
This is where the toggle bolts that cashy explained and has shown, go into.
The problems can occur when you have a "U" shaped Kitchen, then you may need to re-cut a masons mitre.
You need to lay the tops in position and scribe against the walls if they are too far out!
Next lift each top and seal the end grain with paint or silicone. These flatpax come from China and their HMR tops do not have the same qualities as that which you get from local top suppliers)
Replace the tops into position and insert toggles into the holes (May need to cut part of the carcass out to access holes)
As you slowly start to tighten the toggles, place a bit of timber on the tops and tap firmly with a hammer to get the join level with eachother. You are trying to ensure there is no lip at this point!
Once you are happy there is minimal lip (you will hardly ever get a perfect join) tighten the toggles the remainder of the way.
Hope this helps you
cheers
Adrien Mamet
www.mametconstructions.com.au