It's a bit of tap dancing. It's using the existing dialogue tracks for the most part, replacing some of the production sound (things like breathing, movement, etc.) with cleaner versions from the ADR lounge, and then trying to make it match the soundspace. Adding hits to a fight scene, using them to enhance the existing sounds, just to add a little more oopmh and reality to the hits. Appropriate grunts when an actor gets hit. Replacing the sound of the plastic gun getting kicked away with a sound of a "real" gun skittering away on concrete, which I had to create from scratch.
Creating a more realistic gunshot sound. Even has a tiny shell eject in the tail.
I think I'm about halfway through that one, maybe a bit more.
In a way, this is where the art comes in. Trying to get just the right sound, tweaking the balance of sound elements, layering sounds to achieve more of an effect and create new sounds entirely.
The hard part is having to decide what to use in order to save time. For instance, I was doing some foley work earlier this morning to try and duplicate the sound of a sword hitting the floor. Well as it turns out, a couple of the sounds that I produced make dandy sounding fight hits, so the little fight accents get a sprinkling of these. It helps that I know the character of the sound that I want when I see something, but I think that comes from experience.
But there's a limit to what I can do before my judgement comes into question, and I hit that point. I can usually do two good hours before I need a break, but there's a cumulative effect and I just need to stop and take some downtime. In this case, sleep.