It's impossible to lose only arm or leg fat unless through a surgical or laser specialist. Therefore, the method is to lose fat period. Build the muscle in the areas you want to target, but it's best to do that over the entire body as well. Don't want to be lopsided. The harder/larger muscle will tighten up the fatty areas, and the full-body fat loss will thin up the skin. This combo will give you what you're looking for. Remember though, opposing muscle groups are important. If you feel fatty in the bicep area, do bicep curls. However, doing tricep extensions grows the tricep, which in turn, pulls the skin around it, making it more taught. Common sense tells us that this also makes the bicep skin taught, as the muscles are getting larger in opposite directions. Food for thought.