Here is a link to the Google Document on how to install Kali Linux on Virtual Box. Below are the same instructions without pictures. If you need help with these steps feel free to reach out to us. These instructions will specifically be about installing Kali on Windows but the steps for doing this on MAC OS are similar.
Download the Kali Linux version at https://www.offensive-security.com/kali-linux-vm-vmware-virtualbox-image-download/#1572305786534-030ce714-cc3b
Make sure it's the virtual box version. Preferably the 64 bit version unless you have a 32 bit computer
Place the file you downloaded somewhere on your computer where you won’t change it. I recommend making a folder called ISO’s or VirtualBox Images.
Download VirtualBox on the VirtualBox website
https://www.virtualbox.org/wiki/Downloads
Make sure you download the correct version for your OS
Install Virtual Box by running the installer you just downloaded.
Run Virtual Box
Once you run the program you'll want to click on import on the top bar so that you can add your Virtual Machine
Click on the folder icon to select your Kali Offensive Security ISO
Navigate to where you saved your ISO. It should end with .ova and select it
After you’ve select it and click next.
From here you'll just select import and let it import everything
From here you should be able to start the virtual machine by clicking start at the top after selecting it or double clicking your chosen virtual machine.