Re: Setting up Hornady resizing die
Follow the directions that came with the dies. That will get you set up to fully resize your brass.
The die has a "chamber" cut in it just like your rifle's barrel. With the shellholder touching the base of the die it acts just like the chamber in your rifle with the bolt closed and locked...it is a fixed size and can't size the brass any smaller to cause headspace problems unless your rifle has an oversize chamber.
Hornady rifle dies adjust the same as every other brand of "standard" rifle dies. RCBS, Lee, Lyman, etc... the basic concept is the same across the board until you get into some specialized sizing dies like neck sizers, collet sizers, etc.
To adjust the decapping stem on the Hornady dies, loosen the locking nut and press/turn the stem up or down as needed. You want the decapping pin sticking out the bottom of the die but you don't want the expander portion of the stem to protrude beyond the base of the die.
If you have any more questions, feel free to post them up. And welcome to TFF!