Add two blocks of fat (that's about 3 grams of fat) for each block of protein, in other words double the usual amount of blocks you'd add.
Here’s how Dr. Sears explains fat blocks (taken from MASTERING THE ZONE, page 292 and 293). It will give you a better understanding.
“Why is a fat block only 1.5 grams?
Every block of low-fat protein contains approximately 1.5 grams of “hidden fat”. Therefore, by adding one extra fat block (which is defined as 1.5 grams of fat) for each block of low-fat protein, you are actually consuming 3 grams of fat or two blocks (one internal in the protein and one external) for each protein block. If you are using fat-free protein sources, such as isolated protein powders, then you should be adding two blocks of fat to achieve the same ratio. Obviously, if you are eating higher fat protein choices, you would not be adding any extra fat blocks to your meal. Remember that every time you add additional fat blocks to a meal, they should be composed primarily of monounsaturated fat.”
Common fat free proteins eaten on the Zone diet are egg whites, protein powder, some deli-style meats, and some soy products.