Clip off a 1 inch length piece of foam. Foam width is determined by hook size and personal preference. Hold end of foam on your side of hook at an a downward angle. At extreme end of hook is where we want to tie in wing case material. Make sure there is not any foam material past middle of hook shank. Too much material forward will mess up forward part of fly.
Make a loose wrap of thread.
Make another loose wrap of thread.
Tighten thread toward your belly button, this will roll wing case material on top of hook shank. Do not tighten too hard or thread will cut foam.
Make another couple of wraps and tighten harder. Repeat this process going forward toward hook eye until foam butts are lashed down tight.
When you get to the end, look at rear of hook shank and see if there is a gap between where the wing case is tied in and the rear end of hook shank.
If there is a gap, wrap tightly back toward rear of hook and close gap.
Tied in wing case should look like this. Ready for:

Peacock body