There are two basic tactics when fishing the "Washing line", or multiple fly leader setups. Either you fish it vertically, ie, to find the depth at which the fish are feeding at. By doing this, you will be fishing accross the water column, or vertically to the water column. Once you have found the depth, you could then choose to fish it horizontally, along the water column, at the depth you hvee found the fish feeding in. You will find the correct depth of the feeding fish, by using different sinking and intermediate lines, by counting them down according to the descent index.
If you have a 1.5 intermediate, it will sink at 1.5 inches per second, so you know that if you count down 30 seconds, your line should be at 45 inches. If you are fishing a DI 5 sinker, you know it sinks at a rate of 5 inches per second, so a count of 30 seconds, means it will be at a depth of 150 inches or 12.5 feet or about 4 meters.
There is also no reason why you cant find the fish by fishing it horizontally. I would do this by counting down every cast, say first cast, count to 5, next cast to 10, and so on and so on, until you have action. the next 5 or so casts, do the same thing, to confirm that the fish are at that level, or if it was just an isolated "patrolling" fish.
Successfull stillwater fishing , is an absolute art, and there can be a lot of science attached to it as well.
Here is a site that might help
http://streamlinedflyfishing.com/tec...n-and-retrieve