The timing of when to use these items is actually pretty critical since they're a limited resource. For the best results you want to wait until you see the boss's initial attack animation, which usually happens right as you cross through that gold fog wall. The moment you activate a rune arc you'll get those stat boosts or special effects from whatever great rune you've got equipped, and they'll stick around until you die. Worth noting that when you're running godrick's rune, you'll get that +5 to everything right as the activation happens, so time it before attempting any specific weapon requirements or spells.

What's really interesting about the positioning is that having those extra stats from morgott's or radahn's rune can completely change how aggressive you can be, especially during those moments when bosses do their big area attacks. The extra health buffer lets you stay in closer, and get more hits in during recovery frames. But here's the thing - you need to watch the boss's wind-up animations extra carefully because even with the buffs, those heavy hits can still one-shot you if your timing is off.

Definitely keep an eye on your stock though since they're pretty scarce early on. Maybe interesting - helping other players with bosses is actually one of the most reliable ways to farm these, and you'll want a decent stack before taking on the later game areas where every bit of extra stats really counts.