There's a correct way and an incorrect way to do this. The incorrect way is to put the scale's hook under the gill and attempt to weigh the carp like you would a largemouth bass. That will almost surely cut a gill and kill the fish. NEVER stick a weighing hook into a carp's mouth or gill plate.
Here are a couple of ways to do this safely. Most of the serious guys use a weigh sling. This is basically a sack made of fish friendly material. The sack has handles. Weigh slings can be purchased at most online stores. Put the fish in the sack, and put the handle onto the scales's hook and weigh. Check out my friend Jerome at right properly weighing a carp. If you want to be precise, subtract the weight of the sling. Another way you can do it is to leave the fish in the net and hook the scale around the net and weigh. Once again, later weigh the net and subtract that weight to get an accurate weighing. One more thought...get a decent scale. Berkley makes some high quality digitals at reasonable prices. Rueben Heaton makes the top carp scales in the world (find them at Wacker Baits), but they are expensive.