I bought two things off Amazon.com to be able to do this. (I will provide links to the products at the end)
So it's real simple. !) Funnel in the bottle. 2) Pour the paint in. 3) Add your thinner of choice (I use Liquitex Airbrush Medium, a clear acrylic medium meant for watering down acrylics without having them "break"). 4) Add a drop or two of flow aid. 5) Shake it up! It's really that easy.
Now I just have to add a few drops into the gravity feed on my airbrush and I'm good to go. So here are those links if you're interested: