Familiarize yourself with the proper use of a wet saw and confirm that the saw is working properly. This includes making certain that the water sprayer is spraying water on the blade whenever the saw is running.
Measure and mark where you wish to cut your tile. Small strips of masking tape can be used to mark the line of a cut.
Place your tile on the wet saw's cutting platform. Make certain the water sprayer is working and slowly bring the blade down to cut your tile. Wear gloves and eye protection whenever using a wet saw. Keep fingers away from the spinning blade.
Use your tile snipers (which look somewhat like tongs) to grab and snip off tiny pieces of tile that the blade cannot easily cut. You will have to exert quite a bit of pressure on the tile snipers to break pieces off of a marble tile, but it can be done. Break off small pieces only. If you have to break off a large piece take it in small "bites."
Sand the edges of your cut tiles. Start with your 80-grit paper and sand until you have a smooth edge, somewhat beveled. Then use your 120-grit paper to smooth the edge further and finish with your 400-grit paper. When you are done your cut edge should be almost identical with the non-cut edges.