I did this about 18 months ago in my kitchen. The old formica was in
horrible shape, so I bought two sheets (4x8 each, $44 a piece IIRC)
from homedepot.... also bought some of their contact cement stuff...
it's in the same section as the formica.

Here's my point, mine is starting to 'buckle' in one place. I sanded
the old formica with a 60 grit on a belt sander to rough it up.
Cleaned it with a tack cloth, then pulled out my router with a straight
cutting bit (this is the ONLY way to cut formica, using a hack saw will
take days and you'll probably hurt yourself, formica's sharp stuff
sometimes...but it's possible)

After dry fitting all the pieces together, i pulled them off, then
started putting down the glue. That's where the problem is, I figured
I'd scimp on the glue in a few places as I was running short. (bought
the quart, not the gallon..not smart) PUT THAT STUFF ON THICK. I put
it on super duper thick in some areas and it's a great counter, where
it was thin...that's another story. I hate my bubbled countertop, if
only I did it right the first time.

So, moral of the story is...buy too much then return what you don't
use. I should've gotten 3 quarts and returned any unopened ones, and
slathered that stuff on until it oozed out of the edges.