In SotC every boss had a specific way you had to climb up them. (some on their left foot, others on their sword, etc. Sometimes you had to bait the colossus to swing his sword at you, then jump on it before he raises it off the ground.) Then you just climb around, limited to the spots that have fur, unless it's level and you can walk. Then you stab the tattoo when you find it.
I guess the best way to explain both quick time events and button sequences on bosses is the good old "press X to not die". If you mess that up, you're either dead, or you have to start over on setting up the combo. Without that, you could just shoot again if you missed.