Instructions

1

Make sure you are not liable for any damages. State and local laws will impact your liability and it is advisable to check your regulations before cutting anything. Most states allow you to cut back branches at your own expense to your property line only, and, in some cases, providing it does not permanently damage the tree.
2

Talk to your neighbor and get a disclaimer in writing that relieves you of any responsibility for the health and welfare of the tree or trees in question. Matters such as tree trimming are best resolved personally and, preferably, amicably. Sometimes the best policy is to exchange disclaimers.
3

Call in a certified arborist. This will ensure the job is done properly, minimizing any negative effect.
4

Learn to make cuts properly, if you'll be doing it yourself. Always prune cleanly and on a slant to keep water from collecting and infecting the cut. If possible, cut back to healthy wood just above a node or branch. Find a slight collar or bulge where the branching takes place, and cut smoothly on the same angle as the collar and cut on the outside. For root issues, cut out the cement and cut the root back before replacing the cement.
5

Document the trimming with pictures, whether the cutting is done by an arborist or you.