Can a priesthood holder with disabilities perform a baptism?
Absolutely! The Church Handbook of Instruction (vol 2, 20.3.8) lists the steps for performing a baptism. Assuming you’re the one baptizing, you:
1. Stand in the water with the person getting baptized.
2. Hold the person’s right wrist with his left hand (for convenience and safety); the person being baptized holds your left wrist.
3. Raise your right arm to the square.
4. Give the baptismal prayer (D&C 20:73).
5. For convenience, the person getting baptized holds his or her nose; you will then place your right hand high on the person’s back and immerse him or her completely.
6. Help the person come out of the water.
You are only required to raise your arm to the square and assist the person back out of the water. The other requirement is that the prayer is said. Anyone who can do those things can perform a traditional baptism regardless of physical or mental abilities. You may even recall that the Holy Ghost performed the first baptism without a body!
The first volume of the Church Handbook of Instruction contains further direction on adapting this ordinance as needed. Speak with your bishop and he can tell you the nuts and bolts of how this applies to your particular case.