Hi Ellie. From what I am reading, you want the two tops to meet against the walls and to be 100% spot on where the joint ( 45*) meet. simple. ok, here we go, now on your left you have a top. place that to the wall, with the end you want to cut in the courner. now, the other one which will be for the left side you place that againest the wall, and let it overlap the first one, make sure that the back edges are to the wall, and the 90* on both are in line ( on top of each other) then look to the front ( thats the part you can see) and mark the spot where they overlap each other mark the top one and the bottom one at the same time, take a nice striaght board, and mark a line from the back ( 90*) to the front ( where the small line is) " but make sure it is straight ( and not micky mouse ) you then do the same to the other one because you have also a line on that one too, cut them both. and place them together, if you ahve a "shall we say a small space" you can still plane that away. till they fit 100%,also make sure you keep the formica edge has a sharp edge ( so take care when you handle it) some people glue the joint, some people put silicone between the joint to stop the water getting through. the chose is yours. well anyway thats how its done, have fun but double check before you cut the tops John 2000.