what is the best(correct) way to set up a full lenght resize die to headspace on shoulder instead of belt?

Adjust the die so that the case will just close in "your" rifle. This way you are setting it on the shoulder, not the belt.

The first firing the case will headspace on the belt. I take a once fired case and back the sizer off and run the case in. Slowly lowering the die until I just touch the shoulder. Then try the brass in the rifle. If it chambers fine if not I will move the die down a little more.

Usually the first time or two it will chamber. The the brass will grow enough after firings that you need to lower the die a little.

If you want to headspace on the shoulder from the start. Take your brass and expand the neck to 35 cal. Then back your 338 die off and slowly lower it until the brass will just chamber. Then I usually turn the die down about a 1/4 turn.