If you already have a will, and have decided you need an update, make sure to write out your comments and questions on separate paper. Don’t write on your original document! Writing on a will invalidates it!
It is always best to keep your original document clean and unaltered. Keep it away from stains and moisture and don’t unstaple the pages.
If you have a reason to make a change in your will, or you want to mark certain passages for your own use, make a copy of the original (without unstapling it) and use that copy to write on. You can then take it with you to your lawyer’s office. By keeping your original will unaltered by your pen or pencil, you are covered by that will until you get your new one drafted and properly signed.