First you have to get your reaction equation right.
I'll use a simple example. You can make Iron and Sulphur react to form Iron(II)sulphide. Or in formula:
This means that one atom of Fe will react with one atom of S.
Now look up the atomic masses of both, you will find 56 for Fe and 32 for S. This means that Fe and S will always react in this mass proportion, whatever the weight/mass units we apply.
So 56 grams Fe + 32 grams S react in proportion.
But it could also be 7 + 4 (same ratio), or everything in ounces.
You do it like this:
Write out the equation, then put the numbers from the Periodic table underneath each ingredient, then in the line underneath that, write the number that you know. The one you're missing can be found by using the same ratio.