I have a lemon tree that is 8 years old. I didn't water it enough during its first years, and as a result it's only 2 meters (6 feet) high. How and when should I prune my lemon tree to stimulate its growth?
Depending on your location you will want to prune any time from January to March (or June to August in the southern hemisphere). If you are in Florida a good time to prune is generally end of January or beginning of February. Look for any dead branches and get rid of these first.
You will then want to trim branches that are towards the middle of the tree, that way you allow for more sunlight and the flow of air, this all promotes healthy growth.
Do not prune branches that are healthy, there is no reason to do that. You will want to prune only if the branch is dead, it takes away sunlight from the lemon tree, or it forces the tree to grow in an awkward position.
There is a youtube video on it.
I would prune the tree in early-ish spring, whenever that is in your time zone, since that gives new shoots (which you want) time to ripen in preparation for winter or hot weather.
As JonH said, just prune for shape and structure and health: maybe have a couple sturdy branches to establish the frame, etc. etc, have an open center, get rid of diseased branches or misplaced branches... You wouldn't want to prune too much though, as citrus doesn't respond well to too much branch loss (15-20% max).
Another thing you could do is remove the flowers/young-fruit to encourage growth in other areas of the tree.