Don't worry; it's not that hard. You can use an ordinary circular saw, equipped with a diamond blade, to wet-cut your marble. Because this will be a messy job and you'll need your garden hose, you'll want to work outdoors.
Protect your marble slab by placing it on a 2-in. thick piece of rigid foam. Mark your cutting line with a marker on a wide piece of tape.
Using a hand clamp to hold the hose in place, direct a trickle of water on the front of the diamond blade to cool it. Put on your eye protection and make your cut. You'll be surprised how quickly the marble cuts using this low-tech method. The edge might be a bit rough, but you can smooth them using an orbital sander and slowly working your way from 40 to 400 grit sandpaper.
Expect to pay about up to $120 for a 7-in. diamond blade and about $40 for a GFCI extension cord. These items are available at home centers.
Because you'll be wet-cutting, plug your saw into a GFCI outlet.