Please refer to this article for most of the information.
files:
Steam main body Steam.app
Steam with Family Beta (2 in 1 already packaged as dmg)
Instructions: After extracting the Steam main body, drag it into the Applications folder.
For the App package, in Finder, go to the folder
~/Library/Application Support/Steam/
If the folder is not there, open Steam once to generate the folder or cd to Application Support to create a Steam folder.
Extract the Steam.AppBundle package, then copy the Steam.AppBundle folder to the Steam folder above, and you're done.
For those who downloaded the Family Test package, after completing step 1 above, please pay attention to the Steam inside the package (one is the App package, the other is the Steam main body).
To distinguish between them, click on them and then right-click or click on the small gear in the Finder window. If the menu shows an option called Show Package Contents, then the selected one is Steam.app.
Manually create a folder named Steam.AppBundle in the Steam folder, then drag the Steam icon file that is not the Steam main body into the Steam.AppBundle folder, and you're done.
QA: What to do if Steam sometimes doesn't display windows?
Force quit directly (usually only in the FB version of Steam)