How can I turn an old Macbook pro into a VM to run on a PC in Virtualbox?

I have an older Macbook (non-retina) that I don't use very often because it is painfully slow but I do need it once in a blue moon. I'd like to tu it into a VM so that I can run it in VirtualBox on my PC. I'm currently using VirtualBox for my older PCs that I've converted to VMs so that is why I reference VirtualBox.

I've googled the topic but results seem to point back to creating a hackintosh and that is NOT what I'm after. With a PC, it's very simple and I'm hoping there is a simple way to do it with my Macbook, too. If not, I may just replace the HDD with an SSD to give the old girl a new lease on life.

I'd appreciate any links, youtube video, guides or better yet, personal experience in how you accomplished this goal.