A lockbox on the side of the house is useful only if the people who need it can reach it, it cannot be pulled off the wall, and the code or key works the first time someone tries it. Most problems come from skipping the first step. Do not mount a box until your father has agreed to where it goes. A box he did not choose is one more argument at the side door, and a box he chose is far more likely to be used.
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Step one: pick the spot
Choose a spot on the wall or post beside the side door that is out of the weather, not hidden behind a shrub or a gate, and reachable from the ground without a ladder. Stand where a helper would stand and check the reach at that height. Check whether your lease, condo rules, or association rules allow a mounted box at all. If they do not, the box stays off the house and the keys go to a neighbor or the locksmith on file.
Step two: find the solid surface before you drill
Brick, block, wood siding, and metal posts each take a different anchor. Tap the wall to find solid material, and look for the framing or post under the siding. Use a masonry anchor for brick or block, a wood screw into solid framing for wood, and a through-bolt or clamp for a metal post. Do not rely on hollow siding alone, because a box will tear out under weight. Before drilling, look for wiring or gas lines in that section of wall. If you are unsure what is behind it, stop and have someone who does this work check the spot first.
Step three: set the anchor and mount the box
Drill the pilot hole to the size the anchor manufacturer lists, not to the size of the box. Set the anchor flush, mount the box, and tighten the fasteners until the box no longer turns. Pull on it with both hands. If it moves at all, the anchor is wrong, so pull it out and start the step again rather than tightening harder.
Step four: set the code or load the key, then test it
Set the code or load the key following the instructions that came with the box. Close it, scramble the dial or turn the lock, then open it again to confirm the code works. Test it from outside the house with the same code you plan to give the helper. If it jams, do not force it. Read the reset steps and try again. A box that fails on a cold evening is the worst time to learn how it works.
Step five: log it
Write down the box location, the names of the people who hold the code or key, and where any spare is kept. Keep that note with your father's papers and give a copy to the helper, but do not write the code on the box or on a sticker near it. Change the code when someone who knew it no longer should have it, and write down the change.
Next move: Stand at the side door tonight with a tape, mark the height where a helper could reach from the ground, and tap the wall there to find solid material before you buy any box. Then ask your father to pick the spot from the marks you have made.