A door chain looks like a small job, and it is one, but it is the kind of small job that either works for years or quietly fails the first time someone needs to get out. The install takes little time. What matters is whether the chain and viewer end up where the person inside can reach them, and whether they release without a struggle.
Local option: New Plan Care gives families in the St. Louis area in-home help so a parent can stay home: companionship, help with bathing and dressing, meals, and overnight care. Call (314) 405-0887.
Before anyone drills
Stand inside the front door and open it fully. Note which way it swings, whether the hinges sit on the room side, and whether the frame is solid wood or has been patched. Look at the door edge and the frame near the strike plate. A chain track needs enough sound wood to hold its screws. If the door is thin or the trim is split, say so now, because a chain that pulls out of rotten trim gives a false sense of safety.
Then decide who holds the drill. You can do it on a visit with a screwdriver and a helper outside, or the local circle can do it for you. Ask the installer three things: whether the screws will bite into framing and not just trim, whether they will drill pilot holes before driving, and whether the release will sit where the person inside can reach it while standing and while seated.
Mark, measure, and set the height
Set the viewer at the height of the person who will look through it most often, not the height of the tallest adult who visits. Mark that height with painter's tape before making any holes. Hold the chain track along the door edge, keep it level, and mark each screw. Then close the door and mark the receiver on the frame so the chain slot lines up with the track. Close the door again, check the alignment, and adjust the marks before you drill.
Drive the screws into the frame
Use the screws supplied for the track and the receiver. Drill pilot holes first, then drive each screw until it is snug and stop. Overtightened screws strip the wood, and the chain loses its hold. If a screw spins without biting, pull it, move it slightly, and try again. If the wood keeps crumbling, stop and have a carpenter look at the frame before you go further.
Test it from both sides
Close the door, slide the chain into its slot, and try to open the door a few inches. It should stop cleanly. Then release the chain from inside with one hand, without tools, without a key, and without looking down. Ask whoever is outside to knock, and check the viewer: can you see a face clearly at the height you chose? Repeat the release with the door partly open and with the chain set, the way someone who is slow or unsteady would have to do it.
Keep it safe to leave
A chain slows an unexpected door. It is not a lock, and it should never make leaving the house harder. Keep the release reachable, keep the route to the exit clear, and never mount the chain so that a second person is needed to undo it in an emergency. Write the screw sizes, who installed it, and the day it was tested on a card in the folder, so the next person who touches it knows what was done.
Tonight and the doctor
Tonight, stand inside and practice the release once with your eyes closed. If you cannot do it that way, the person living there may not be able to either. Then send the local circle a photo of the track and the viewer position, so they can check the install without guessing. For the next doctor visit, bring one question: can the person living there reach and work the release safely with their current hands and balance, and is any help needed to get to the chain? Bring the question, not a diagnosis. If you need someone to do the install in St. Louis, start with Local Help for a St. Louis Parent.