This document gives your Successor, Trustee. Or Executor specific and detailed instructions regarding your wishes after you die or become incapacitated.A letter of last instruction as an essential part of their estate plans. It's not a replacement for a will, and in fact it's not even a legal document. A letter of instruction can complement an estate plan and provide your loved ones with additional details about your assets and wishes after death. I am sorry for your loss. Yes, you can keep you dog at home with you overnight. What goes in a letter of instruction? A letter of instruction is a document that lists all relevant financial information that your family members or executor need if something happens to you. Providing for your Pets in your Will – Sample Will provisions and Pet Trusts.