The more simple potions are just medicinal/harmful herbs (depending on the use) put together in an elixir.
The meanings and associations of herbs and such play a big part in them.
It is very easy to create/come up with your own. Just find a basic table of herbs and their meanings and uses and see what you can do. Practice caution though, make sure to do research on the herbs you plan on using to make sure they aren't something you don't want to use.