I've never made a tutorial before. So it's a bit messed up. There's no reason though that this shouldn't work.
I just wanted everyone to have a way to put tank top straps on, so that they looked professional rather than like they've just been sewed onto the front and the back. Those always manage to pull wrong and the stiching is visible a lot of times. This takes a lot more time though.
1- You can either wing the shape you want for your tank top, or do what I do. I like to use half the existing armhole in the t-shirt and then cut a slight scoop across from there to the other half. That way the sides are still intact. After that I turn it inside out and make sure everything is lined up, pin it and sew up the sides. This step can really be done any way you're comfortable with. If you like to cut up the sides, go for it. I just think it's easier this way.
2- Either use your left over t-shirt fabric, or a different colored shirt and measure out something that's the length of the back portion of the tank, from peak to peak, and the width is 1 1/2 inches. Fold down the top of the pieces to the center, right on the green dashed line, you can pin it like this if you want to so that it's exact the whole length of the piece.
3- Align it so that the folded edge is on the right side of the fabric. Then fold the rough edge down so it's inside your shirt. If you look on both sides of the shirt, the edges (folded and rough) should be aligned with the shirt sandwiched in the middle. Also the edge of the shirt should come right up to the inside of the 'sandwich' you created with the border.
4- Sew up being sure to catch the folded edge of your border, the t-shirt that's in between, and the rough edge of your border. Sew it as close to the rest of the shirt as possible without going over the edge of the border.
5- Do the same thing as in step 4 for the front of the shirt. Then measure the armpit parts and for the strap you can use a bra strap to measure how much allowace you need. Then sew those on the same way, make sure that this trim overlaps the trim on the front and the back. Also if you can line up the seam on this with the side seam it'd be really good. And it's pretty important to pin alot when you're doing this.
ALTERNATE VERSION- You can also do the front part but this time measure from the peak of the front of the shirt, and instead of going across the shirt go the other way and measure the part that would be the armpit, then that same part on the back, double that number, and then add the measurement for the front of the shirt from peak to peak. You'll also need to allow strap allowance. For this you can measure your bra straps and double it. It's important to sew it how the diagram shows or it won't turn out right. It should be sewn the same way as the back. I don't reccomend this version but thought I'd include it. I drew this version in the diagram because it's harder to explain.
6- You can either hem the bottom, do the same thing as you did with the rest of the trim, or take the same colored fabric you used for the trimming in as large of a band as you want, fold it in half and attach it to the bottom of the shirt so it has a nice finished bottom.
This step isn't totally neccessary.