At a request of a friend wanting to start up on double-hander, I have put together a diagram to illustrate how a typical Skagit set up looks like. Some Skagit lines like the Wulff Ambush line comes integrated with running line whereas lines like RIO Skagit Short or Airflo Skagit Compact mostly comes as head only (~20ft) and will need a separate running line.
Every Skagit set-up will need to have a sink tip at the business end to bring the fly down. In the case of lighter Switch rods (4wt - 6wt), RIO versi-leader makes very good sink tips and is a joy to cast.
My preferred set-up for Switch rod is the Wulff Ambush line with RIO Spey versi-leader as sink tips. Here is my recommendation for the length of versi-leader to use for Wulff Ambush line up to 400 grain:
1) Floating versi-leader - 15ft
2) Clear Intermediate versi-leader - 15ft
3) Medium to Fast sink versi-leader (2.6 ips to 7 ips)- 10 ft