Requirements
You will need a host system with Virtual Box installed. Additionally some free disk space is required.
Configuration
Launch VirtualBox and click on New to create a new virtual machine.
Enter the name for the virtual machine (e.g., IPFire). For Type select Linux. For Version pick the line that includes Linux 4.x (64-bit).
Select how much memory you want to allocate for IPFire. Two (2) to four (4) GB is a good value.
To specify a virtual hard disk for the VM choose Create a virtual hard disk now.
Eight (8) GB is a good size for the hard drive for a simple setup when evaluating IPFire.
Click Create (or Continue in the guided/wizard mode).
Network
The RED interface of my virtual machine is connected to my local network.
In this example, the GREEN network interface is a USB to Ethernet adapter that is connected to another stand-alone computer.
Make note of the MAC addresses for both adapters. It will be needed for the IPFire installation.
Running the Installer
Obtain the IPFire ISO image from IPFire downloads for x86_64.
In the Settings dialogue, navigate to Storage and select the virtual CD drive. Click the CD icon on the right hand side and select the downloaded ISO image.
The virtual machine will be configured similar to this:
Click Start to start the virtual machine and follow the steps as shown on the screen.
After the installation, eject the virtual CD image in the settings dialogue.