it's been such a long time since i thought about these dissociation curves but i'm gonna say you just draw the sigmoidal curve and make it shift to the right thus signifying that the affinity of Hb for O2 decreases and as a result O2 can be released to tissues and out of blood. I'm thinking the y axis is % saturation of Hb and x axis is partial pressures or something (don't quote me on that).