The only thing you need to dowload is the game.
To keep everything simple, there's a package containing everything you need.
It is compatible with BasiliskII as well as SheepShaver and Mini vMac.
You can download it at one of the following locations:
Just put the PoP1.hfs file in your BasiliskII folder.
If you're only using a simple boot disk like the System70_boot.dsk file, you should also download this file and extract its content to your BasiliskII folder.