Due to the way the service works, you'll need to meet quite a few requirements to be able to take advantage of it and thwart your local porch pirate. Your Honda vehicle needs to be one of the eligible Accord, Accord Hybrid, Insight, Passport, Pilot and Odyssey trims. Further, you'll have to be a Prime member and located in one of the 50 cities covered by the service.
You'll start seeing in-car delivery as an option after downloading the Key by Amazon app, logging in using your Amazon account, keying in your vehicle's information and giving it your HondaLink credentials. If you do choose to get in-car delivery, you'll need to park within two blocks of your delivery address between 11AM and 3PM. The delivery driver will then locate your vehicle using its GPS location, unlock and relock it all without any interaction from you. While the HondaLink app itself is free, the Remote Services package will set you back $110 per year after a 3-month trial. Key by Amazon doesn't cost anything on top of that, though, so it's a nice perk if you're already paying for the add-on.
Update: The post has been updated to correct that the driver unlocks and relocks the vehicle without customer interaction.