Let's say your dog has a consistent problem on heeling, such as going wide on about turns. I assume you've taught the dog where he should be and that you are doing appropriate and accurate footwork on a consistent basis, but he still goes wide regularly. To address this, I add a leash correction* to the about turn in this way:
1) As I start into the about turn, I use my verbal cue (CLOSE), followed by a leash pop in the direction I need him to move in order to be correct, followed by praise. I might also release after the leash pop if this is very new for my dog. So Step 1 is CUE, CORRECT, [dog responds], PRAISE, RELEASE (if appropriate).
2A) I immediately try the about turn again, ideally going back to where I just corrected him. I use the verbal cue again, but NO leash pop, followed by praise if there is improvement, followed by a treat. So Step 2A is CUE, [dog responds, hopefully with some improvement], PRAISE, TREAT.
2B) Use this step instead of 2A when you want to be able to go on to step 3 in the same session. I immediately try the about turn again, ideally going back to where I just did the correction. I use the verbal cue again, but NO leash pop, followed by praise if there is improvement. So Step 2B is CUE, [dog responds], PRAISE.
3) Repeat the about turn for a 3rd time, again ideally in the location where you gave the correction on the about turn, do the about turn with NO cue and NO leash pop. In other words, like you would do it in the ring: silently. As soon as you complete it, praise, treat if you see improvement, and release. So Step 3 is DO THE SKILL IN SILENCE, PRAISE, TREAT, RELEASE.
We worked on this sequence for lagging on the outside Figure 8 post in a couple of my classes recently, and the dogs were all showing some nice improvements.
Something else I see people struggle with when learning how to use leash corrections fairly: they correct their dog, but then immediately simplify. What I prefer to do is try Step 2A first - I want to see if my dog learned anything from my correction in Step 1. What I view as a correction may not be considered a correction by the dog. If my dog does indeed show improvement - do remember he doesn't have to be perfect right away! - then that tells me the correction was appropriate. If your dog is just as bad as before, then the correction wasn't sufficient OR the environment is too distracting for your dog's current level of training OR the dog just doesn't understand his job yet. This is when it is crucial to be honest about what your dog understands. Most dogs require a lot of repetitions before they really understand how to respond properly to your cues. If you haven't put in the training time to get in the repetitions, be honest and spend more time on your homework before making it harder or adding leash corrections.
To summarize The Rule of Three (how about helping me come up with a better name for this???)
1) CUE, CORRECT, [dog responds], PRAISE, RELEASE (if appropriate)
2A) CUE, [dog responds, hopefully with some improvement], PRAISE, TREAT
2B) CUE, [dog responds], PRAISE
3) DO THE SKILL IN SILENCE, PRAISE, TREAT, RELEASE
I hope that makes sense. Feedback is appreciated.
* I use a quick leash pop with a release as a correction - the strength of the pop appropriate to the dog's temperament, size, type of collar, etc. - enough to motivate the dog to try harder so he can avoid it the next time, but not so much it makes him quit.