There are several ways to make this transition. You can do a dock start, which some consider to be the easiest because it requires no weight shift or dragging through the water, but there's not always a dock nearby to use this start. Another choice is to go straight to the deepwater start where the skier is in the water and is pulled up on only one ski by the boat. And then there's the drop ski method, which involves a combo skiing start and is followed by the skier releasing one foot from a ski and putting the foot in the rear of the remaining ski. Extra equipment is not needed when going from two skis to one ski.
Most combo skis come with one ski that already has a rear toe plate for slalom skiing.
Before putting on your skis determine which ski will go on the drop foot and which ski will go on the forward slalom foot. You'll want to have the drop boot on snug enough that it won't come off during your start, but loose enough that you'll be able to remove your foot from the ski without struggle on the water. This is important because often when a ski is dropped it turns upside down, making it difficult to locate. The boat driver should be pulling the skier at about 21-25 mph. Neither are cheap to fix or replace.