How to create a URL shortner (with python) without importing bittly or other stuff

So I recently became interested into knowing how to create a URL shortener without using bittly or other things, but I am not very good at using python to connect with other stuff. All I know is:

• Checking to see if the URL is available ( Only to see if it has HTTP:// and unavailable characters, nothing to see if the domain is occupied or not. )

All of the other things... I need help with.

By the way, I COMPLETLY do not understand how to do that, so it would be great if you add comments to show me what is going on.