Step 1. Calculate AAUs
First, if it's not listed, calculate the AAUs (Alpha Acid Units) for each hop addition in the recipe.
AAUs = (weight in oz.) x (recipe hops AA%)
Step 2. Calculate the required weight of your hops
Then, just divide the AAUs you got in step 1.
(needed weight of your hops in oz) = AAUs / (your hops AA%)