Rainbow are going to be deep in most lakes. You are going to find them in the deepest parts of the lake that have adequate levels of oxygen. This is usually a layer of water where the temperature change is the greatest and is called the thermocline. A thermocline will reflect sound waves if the temperature change is great enough, so move to deep water and place your depth sounder in manual mode. Increase the gain, (sensitivity), until you see a solid or broken line in the middle of the water column. If your depth finder has the little fish symbols on it, this may take the form of a line of fish around a particular depth. As you move the boat, this line may move slightly up or down, (a foot or so), but should remain pretty close for a given area. If you do not have a fish finder, your best bet is to tie a thermometer to a string and lower it into the water pulling it up every few minutes to check the temperature and drop it 3-6 feet deeper each time. When you find the depth of the thermocline (the layer with a large temperature drop) you will need to find structure at that depth, when you do you will find the fish.