Once you have prepared your floor, it is time to start installing your marble. Start by finding the center of the room. Measure both the length and the width and put a chalk line down the center of each. Where these lines intersect is the center of the room. Now, the next step is very important. Lay out your marble in a test run. This is important because if you end up with a tiny piece of marble at any end, you will need to shift your center line a little to avoid having a tiny edge on one end of your floor. To keep a symmetrical look, have the starting tile straddle the centerline if needed.