I have pre-cut formica to the correct size before. I lay a straight board under it (like a very straight 2x4), then clamp the formica to it, then run the router down the board with a flush trim bit (just like you would use to trim the edge off in a normal formica installation). That is how I get a straight cut with the formica not glued yet.
Then, I apply the contact cement to both surfaces, and let it dry. Don't worry about the positioning!! Just stick a few dowels up under it so you can space it out, then slide the dowels out one at a time. It is no big deal. You don't even have to use dowels, you can use some thin scrap wood strips, some stiff wire, etc.
It is important to let it dry fully, I have put it on before without it drying enough and it didn't stick nearly as well. And, sometimes it is necessary to put two coats of the cement. I also found the oil based formica glue to be slightly better than the water based glue.
Also, it helps a lot to rough up the old material first, especially if it was laminated to begin with. I usually hit it with a sander and some heavy paper, like 60 or 80 grit.
Wade