What you need:
Any shampoo, an alcohol based liquid (I use Sephora daily brush cleaner), a medium sized bowl, and a towel.
I start by filling the bowl with luke-warm water just a small bit so the glue that holds the bristles doesn't get doused with water and cause it to break up.
I then spray the anti-bacterial cleaner directly into the water. I put a small dab of shampoo right on the bristles of the brush and swirl it in the water.
You'll start to see the water get cloudy (or completely opaque depending on the state of the brush). You'll most likely have to change the water every few brushes and then just respray the cleaner. Just keep swirling it and rubbing it with your fingers until the bristles turn to their normal color. Be sure to check more than just the tips of the bristles, because makeup can get stuck deeper in.
When you're finished with your brushes, rinse them in fresh water under the tap and dab them with a towel. Leave them on their side (again so the water doesn't get too much into the glue) and set them to dry. Small brushes take a couple hours where big brushes can take up to a day to fully dry.
Doing all these brushes probably took 12 minutes. It's so easy and mostly uses household materials. Try it out!