My plan so far has been to lose fat in moderate amounts. I found that aiming to maintain the body weight while doing resistance training easily achieves that.

So every week I gain equal amount (in weight) of muscle to the fat lost. The difference is also visually noticeable (increased upper body muscle tone; decreased belly and lower back fat).

In regards to "doing diet and exercise at the same time" (academically referred to as: resistance training under calorie restriction) you may find this article useful: http://cristivlad.com/resistance-training-under-caloric-rest...