You will need a host system with Virtual Box installed. Additionally some free disk space is required.
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. 1GB is a good value.
To specify a virtual hard disk for the VM choose "Create a virtual hard disk now".
8 GB is a good value for a simple setup for evaluating IPFire.
Click "Create" (or "Continue" in the guided/wizard mode).
The RED interface of my virtual machine is connected to my local network.
In this example, the GREEN network interface is a USB to Ethernert 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.
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.