In most houses, one person knows where the water shutoff is, which filter fits the furnace, and when the last visit happened. That knowledge usually lives in that person's head. Your task here is to move it onto paper, in a form the next person can use without you in the room.
Copy the nameplates first
Walk each system and write down what its label says: make, model, serial number, and the manufacture stamp. Photograph each label as well, so the record survives a lost page. The install year you read from the stamp becomes the starting point for every planning window you work out later. If you want to see how a year turns into a window, the gas furnace service life and when to plan page shows the method for one system, and the appliances ranked by typical service life page lays out the comparison across the household.
Mark every shutoff and what it controls
Find the water main, the gas valve, the electrical panel main, and the isolation switch for each major system. Label each one in the binder with a short sentence that says what it stops. Note where the key, the valve handle, or the tool for a manual release is kept. A sketch of the house with arrows is enough. Someone who has never been inside should be able to shut off water in a few minutes using only your page.
Write a visit entry, not a memory
Each time a technician comes, add an entry. Record the visit's season, the work done, any parts replaced, and the filter size fitted. Keep the entry in plain terms, such as cleaned, tested, replaced igniter. Leave out opinions about the technician. The entries are what allow the next planner to judge a system by its history, which is more useful than any single visit.
Check the warranty papers for their clock
Put the warranty papers and any service agreement in a sleeve at the back of the binder. For each one, find when its coverage starts counting, whether parts and labor run on separate clocks, and whether a waiting period applies. Read the cancellation and transfer clauses before you assume either is possible. The figures and comparisons published on this site are in the tables on the index pages, and the contract paper in your hand governs what your own agreement says.
Decide who can open it
Name the person who keeps the binder current and say so on the first page. Tell another adult where it lives, so the household is not waiting on one key holder. Keep a photo copy of the nameplate pages on a phone or a shared folder, and keep the paper copy at the house. When the house changes hands, the binder goes with it. Ask for the seller's records at the same time you ask about the systems, and file whatever arrives beside your own entries.
Review it at the seasonal walk-throughs
Open the binder at the start of each heating and cooling season. Check that the shutoff labels still match the house, that the filter size is current, and that the last entry is the one you expect. The fall furnace walk-through gives a checklist you can file in the same place, so the seasonal work and the record stay together.