Get them to the right building first
Give the full address, the building name and a map pin you have checked yourself. If the postal address is not where a driver actually needs to go, say so. Sat-nav can be wonderfully confident while sending people to the wrong side of a residence, so add one landmark that is easy to recognise on arrival.
A couple of useful photographs are enough. Show the entrance they need, not the nicest mountain view. Think about winter too: a sign, path or parking line that is obvious in September may be half-covered by snow in January.
Unloading and parking are not the same thing
Tell guests where they can stop with the luggage and where the car needs to end up afterwards. If the parking is allocated, shared, underground or height-restricted, say so plainly.
For public or paid parking, link to the operator’s current information rather than copying a price that may be out of date by the time somebody reads it. Then explain the walk back to the apartment. Guests should not have to work out for themselves that the convenient place outside the door is only for unloading.
Explain the bit between the car and the front door
This is where a lot of otherwise good guides become vague. Give the entrance, floor, lift or stair route and apartment number in the order guests will see them. If there are unavoidable steps, mention them before booking as well as in the arrival instructions.
For a key box or similar system, explain the physical action clearly and keep the current code in the private arrival message. Check the box before guests arrive too. The right code is not much comfort when the mechanism is frozen, stiff or jammed.
Have a plan for when access goes wrong
Even a very clear guide needs a fallback. Say who to contact, how to contact them and when that channel is actually being monitored. The person answering needs enough information to help quickly, including the property address and how access is supposed to work.
If there is a backup key, agree in advance who holds it and how it can be made available. "Contact us if needed" sounds reassuring, but it is not much use if nobody is watching that phone during the arrival window.
Make the guide easy to find when it matters
Keep the essential route in short numbered steps with useful photo captions. Arrival instructions should not be buried between restaurant recommendations, ski-school links and the house manual. Give guests something they can save before travelling, with a visible last-updated date.
Then test it. Ask someone who does not know the apartment to follow the guide without coaching. Wherever they stop or ask a question is where the instructions need work. Repeat the test after building work, parking changes or anything else that alters the route.