Fog is a major problem for a lot of airsofters. There is a lot of options to help stave off fogging, but the most cost effective option is a low cost, multi use wipe. These wipes are good for a few hours after you open the bag. The bag includes a white wipe with the anti fog solution and a blue buffing wipe. A paper towel can be used in place of the blue wipe if you'd like. You can follow the directions on the bag, but building 52 recommends a slightly different method. 
Dry goggle of sweat and apply the antifog solution to the inside and outside of your goggle lense and/or glasses. Let it sit for at least 5 minutes. Wipe off the rest of the solution with the buffing cloth or soft paper towel. This should stave off fogging for about 20 minutes and then you need to dry off the goggles and reapply the solution.