At any point during the Resolve Your Roll phase of Gasket’s turn, you must place the Green Bone in the Backup Plan. If you rolled multiple Bones, you can choose which order to place them in. Then, you must resolve the Backup Plan slot that the Green Bone is in, removing it and all Bones to the left as you normally would, and sliding down any remaining Bones.
If the Green Bone is your second Bone, you will spend the two Bones but get no benefit, as there are no Skills associated with the second Bone slot.
If the Green Bone is your sixth Bone, you will resolve the 6 Bone ability of gaining Gasket’s Innate +1.