The first thing is install Packer, go to https://www.packer.io/intro/index.html. Moving on to the Packer scripts themselves you can pick what script you need, as soon as you start the Vagrant box, via
vagrant init hashicorp/precise64
Or whatever box you want to Vagrant into, another example is
vagrant init ubuntu/trusty64
Then make the Vagrant box go up
vagrant up
Once you've done that make sure you have all of the dependecies you need, run
brew install qt
Once you've done that, install VMWare Fusion, these scripts are made for and optimzed for VMWare Fusion rather than VirtualBox, in the Prowl private repo, there are some keys left to get the $80 version of VMWare Fusion, so please look in there if you need VMWare Fusion
https://www.vmware.com/products/fusion.html
As soon as you have VMWare Fusion installed, run
packer build virtualbox.json
Even though it says "virtualbox.json" in the command, it will work on VMWare Fusion just fine. Again, these scripts are optimized for VMWare Fusion, but you can use VirtualBox if you must. If you're not familiar with VirtualBox, read the documentation, which can be found here
https://www.virtualbox.org/wiki/Technical_documentation
These scripts are ideal to create identical images for different environments, to test Prowl on. Which is why these scripts exist.