How do you make a simple paper beam that can hold fifty or more pounds?

Roll the paper tightly. A ream of paper comes in a roll that could easily hold over 50 lbs. If you limit the air between layers the paper beam will be much stronger. Also, consider placing the weight on the end of the paper roll, rather than across the length. A point load on the end of a roll will be held up better than a point load on the centre of a beam.