Adding decimal values is just like adding whole numbers. For the most part you can just pretend that the decimal point isn't even there. Here are a few examples:
.556 .833
+ .433 + .124
------- -------
.989 .957
.412 .621
+ .351 + .371
------- -------
.763 .992
.334 + .224 = .558 .413 + .225 = .638
.453 + .132 = .585 .091 + .102 = .193
Now you get to try a few for yourself.