Once the person has been assessed as lacking capacity to make the decision, the best interests checklist must be followed to make the decision.
Before you proceed, check if the person has an LPA for Health or if they have made an advanced decision for this heath issue and follow relevant guidance.
Check too if the person is entitled to an independent mental capacity advocate? Or will benefit from a general advocate?
Ask yourself again if the decision needs to be made now or if it can wait for a time when the person will be able to be better involved in the making the decision or be able to make the decision themselves.