Cast in and reel up the slack line till the Marker Float knocks up against the Lead/Sinker. Now use the same technique as checking the bottom by lifting the rod up and dragging the Lead/Sinker on the bottom. If you are using a 12 ft Rod then you are dragging back 12 Feet at a time. Reel the slack up till you feel the Marker float knock up against the lead/Sinker. Then Pull line off 1ft at a time and count the ft till the Marker Breaks the surface.
Note that in the Diagram That the Angler has pulled back 12 Ft. So at a Distance of 13 feet it is 3,5 Feet deep. Once you have located the Drop off and you are happy with the depth and distance you now clip the line in on the Spool.
At this point you want to make sure what the Distance is, so reel the line in. How to check the Distance and to make sure that you will be casting on the same spot every time is easy from here on out.