The idea of a habit stack is that our normal thoughts about habits generate ugh fields by virtue of just making us feel bad about some laudable habit that we don't yet have. Instead, we would like to reflexively bundle together the habit, its implementation intention, urge propagation related to why we want the habit, some incentive analysis to proactively eliminate trivial inconveniences, and some use of memory techniques for making the habit available in the right context. A bare habit is just a tool for making us feel bad, like a knife without a handle.
Pairing the cue to exercise with the cue to eat, and rewarding exercise immediately with hedonic foods.
Pairing the urge towards time wasting websites with the urge to urinate, building in a natural time bound.