On to the tank top. . .
The best thing to do will be to find a tank top of your kiddo's that you like the fit of and use it for a pattern. If you don't have one handy, measure the kiddo. First around the chest, just under the armpits. Then figure out about what length you want from the top of the tank to the underarm seam. (This is only a couple inches-- it was almost the same for my 2 year old and my almost-6- year-old.)
Trim the ends of the binding to match the tank, like so:
Cut the strap pieces to the exact length you want them. For my almost-6-year-old, they are about 15 inches, for the 2 year old they are about 13 inches.
To attach the straps, start at the underarm seam and pin the strap onto the tank with the elastic side on the right side of the fabric.
Then take the opposite end, fold it over about 1/4 inch and pin it into place just over the already pinned end. Continue pinning around the entire strap, keeping the serged edges even all the way around.
Zigzag on top of the serged edge all the way around, removing pins as you go. Again, make sure you catch the under edge of the strap as you go. You will also want to stitch a little seam right over the top of where you overlapped your strap pieces under the arms to keep it in place.
If you want a hem band, attach it the same way as the top binding, matching the side seams.
If your band length is shorter than the hem length, pin the band in place on the side seams and at the center points of the front and back, then stretch slightly as you stitch around to take up the slack.
And there you are!
An easy, cheap, adorable tank perfect for layering!