There are many resources available most of which advised to do several steps and repeating them all over again. But what I followed was using vinegar and hot water. Hot water will kill the mildew and so will the vinegar. I thought that's a powerful combo without the discoloring effect of bleach.
Here's what I did.
Part 1
1. I boiled a pot of water then poured it into a small basin.
2. I mixed a cup of vinegar with the water then immersed my t-shirt in it.
3. I placed a heavy glass plate to press down the t-shirt to fully submerge it in the mixture.
4. I left it soaked for about 3 hours (mainly because I was doing other things. Others say 1 hour is enough. But I wanted to be sure also.)
5. I washed the t-shirt in the mixture by hand targeting the areas with lots of mildew. (I noticed the mixture getting dark in color. My t-shirt came-out free from mildew after this.)
Part 2
1. I rinsed the t-shirt to remove the vinegar smell.
2. Soaked the t-shirt in a detergent overnight. (Because I was sleepy and tired already. 30 minutes should do it.)
3. I washed it to make sure no stains are left, then rinsed it.
4. Dried it under the sun to make sure any mildew pore residue would be killed.
It actually is simpler than I had enumerated. Just 2 parts: Washing with hot-water-vinegar solution, then washing with detergent. This is a good option if you have asthma or are allergic to mold and mildew like me. Most advice to scrub the fabric outside of your house, but that wasn't an option for me because of the spores that I could inhale. That's why I soaked the t-shirt without pretreating or brushing it.