How do you make loading bearing beam out of 2x12?

Usually it is in a 2x4 wall. Take two 2x12's the right length and put a strip of 1/2 inch thick plywood between them. Nail through from each side at regular points alternating the spacing on each side. This ties everything together. The plywood and the different grains of the 2x12 give it more strength than the parts separately.

good answer above. make sure to crown both 2x12's in the same direction and adding construction adhesive to plywood before nailing all together gives it even more strength. Install it with crowned side up naturally!