This project cost me less than $5. I bought a package of Rit Dye for $1.99 and a clearance man's white T for $2.49.
I mostly followed the directions on the box but tweeked it a bit to get the look I was wanting.
Clean out your sink and add about two gallons of hot water. Pour half the dye packet into the water.
Hang on to the top of the t-shirt and dip your WET t-shirt into the dye up to the armpits. Swish it a bit while keeping it in the water for a couple of minutes. Don't over swish or you will get wave marks in the t-shirt. See my mistake!
At this point I realized I didn't quite care for the white-white with the teal so I filled up the sink almost to the top with water to really dilute the dye. I dunked the entire shirt for about a minute and then pulled out. Ring it out and put on a garbage bag on your counter to dry out for a few hours.
Toss into the washer with a load of something you don't care about (towels, in my case) and wash with laundry soap. It will fade. . . a lot. I believe that leaving it the dye longer will make the color darker but I wasn't looking for anything too dramatic.
If I do it again:
1. I'd be more careful when dying it so that I didn't swish too much. There's a weird spot right around the boobs where it splashed up.
2. I would probably leave the shirt in the dye a little longer. I wanted something sort of muted with this color because teal can get very bright very fast. I would love to do a denim blue color that fades to white.