First, visit the Divine Tower in the region where you obtained the Great Rune (e.g. for Godrick, Tower of Limegrave). Restore power to your Great Rune. Rest at a site of Grace and equip from the Great Runes menu item. Use a Rune Arc to receive the bonus. Effects expire after death. Earn Rune Arcs from co-op play or purchase from Twin Maiden Husks at Roundtable Hold (Table of Lost Grace).