You will need a small syringe without a needle, a 10 mL syringe will do fine.
Warm the mineral oil to 80°F.
You'll have to restrain gingerbread so the best way to do it is to wrap him in a small hand towel or paper towel while you hold him.

Fill the syringe with 5 mL of mineral oil and just push the end of this syringe in the edge of gingerbreads mouth and slowly inject the mineral oil.

Be careful not to push into much too quickly or you could choke him.

good luck!