It's critically important not to cut the plastic bag that is between the inner and outer layers of the glove! If it is cut, or pierced by the needle, or torn, the glove will no longer be waterproof.

Start by carefully trimming the inner glove around the pinky finger. Allow a 3mm (1/8") seam allowance, and also consider that the inner glove needs to cover the end of the joint where the finger would be. Leave more fabric than you think you will need.

Push the plastic bag into the glove. Don't worry about the plastic tab for this finger.

Whip-stitch the inner glove closed to cover the gap where you trimmed the pinky finger fabric.