This is probably the hardest to explain. You have to first find a number that both bottom numbers can be changed to by multiplying.  Then you also have to multiply the top numbers by the same thing.  I know, it sounds crazy, but watch-

 

1) http://www.math-drills.com/fractions/fractions_add_easy_001.pdf

2) http://www.math-drills.com/fractions/fractions_add_easy_005.pdf

3) http://www.math-drills.com/fractions/fractions_add_mixed_easy_001.pdf

And lots more practice at http://www.Math-drills.com