Here is my new tank!
Here is how to use shirts that fit you as your "pattern"
Fold large shirt and fitting shirt down the middle, lay fitting shirt on top of large shirt. With your tailor's chalk, draw on the line using the shirt that fits as the guide. Then add your seam allowance (I used the width of my tape measure at 5/8" because 5/8" is where it's at) Cut on your outside line, cut off your sleeves (save them if you want to have sleeves later).
Consider the amount of stretch in your material too - if the fitted shirt you're using as the pattern is a lot stretchier than the band t-shirt, then you need that ease for the fit you're after. Either adjust accordingly (I just eyeballed it when I cut) or try and use a shirt with a similar amount of stretch.
I wanted to base the collar on this purple shirt. I folded the front of it down the middle while the back fabric just kind of hung out (otherwise you can't trace the front collar if the back of the shirt is in the way). Chalk on the line, add your seam allowance. Do the same for the back.
I just pinned on the sleeve, then leaving the pins in lifted up the purple shirt and put a chalk mark at each pin. This gave me a dotted line for my sleeve. It helps to have a padded surface to pin into.
Now you cut it up and sew it together, finishing in whatever way you like. I left the original hem because I liked the length. I also decided to do a tank top, although I saved the sleeves thinking I might do a little cap sleeve or something. When I decided not to I just did a rolled hem instead.