To install Hello Neighbor, first go to the download button at the end of this text and click on it. You will be taken to the download access page, where you will have to click on the green button:

You will arrive at the download access page, where you will download not the Hello Neighbor installer, but the Steam client installer:

If you do not have the client installed, double-click on the file and follow the instructions on the screen. If you do not have an account you will have to register, if you already have an account, it is more than likely that you already have it installed, so you will just have to log in. In our case, we already had the Steam client installed and we already had an account created.
If this is your case (we assume this is the case for most users), from the store page, in the search engine, type "Hello Neighbor" and press enter:

You will get to the game's page in the store, where you will have to buy it (we already have it in our library):

When you have purchased it, the Steam client will give you the option to install the game. To see the progress, you will have to go to your library's page:

You can see more detailed progress of the download by clicking on the Downloading button at the bottom of the window, which will show you the download speed of the game and the installation progress:

When finished, the Steam client will inform you that you can now play the game by clicking on the Play button:

Besides this, the whole process is fully automatic and takes place with little or no user intervention.