Do you mean a 15' floating tip? A skagit head is ideal for casting sink tips and streamers with the setup being skagit head -> sink tip -> 3-4' mono -> fly. On the bow a 15' type 3 sink tip is a good place to start and 3-4' of 10lb Maxima. Cast down and across, let your fly swing in the current (make sure there's no slack in your line so you can feel any strikes) until your line is parallel with the bank. Let the fly dangle a second, then strip it back in and recast. In faster or deeper water you can cast a little further upstream to let your fly sink deeper. Experiment to figure it out. You can also try stripping your fly during the swing. One way I cover water on the Bow is to make a cast, let it swing to the dangle, strip in, recast. On this cast I start stripping in during the swing. Take two steps and start over.
I'd highly recommend taking spey casting lessons from a qualified instructor. By learning off a video or book you risk developing a lot of bad casting habits that will be difficult to break later on. An instructor knows both how to spot those faults and correct them immediately before they become ingrained.