You rewrite them so that they all have the same denominator.
eg. 9/4 +4 1/4 + .50
convert it to :
2 1/4 + 4 1/4 + 2/4
you do this because it makes it easier to add the total up when you have the same denominators on the bottom.
the next step is to add them all up:
2 1/4
+ 4 1/4
+ 2/4
-----------
7