In short, assuming your existing countertops are laminate (as opposed to butcherblock or something), there are a couple of things that you need to be sure of. One is the condition of the EXISTING counertops, and two, bonding the NEW countertop to the existing one.
You may find books out there that talk about how to do this job. It's important that you find out HOW to bond the new top to the old one based on YOUR specific (old and new) countertops. I have a couple of links to get you started.