grab a big t-shirt and a buttondown shirt. it would be swell if you have a tank top for tracing.

instead of the buttondown shirt, you could also use fabric or another t-shirt for the yoke.

lay the tank top on the t-shirt, and mark like so.

cut where you've marked, but only cut the scoopneck out of the front.

so the back piece should look like this ------->

sew the front and back pieces together.

save that piece that was cut out to make the scoopneck. that will serve as a pattern for the yoke.

lay the t-shirt yoke pattern on top of the collar of the buttondown shirt. cut around it as shown.

using what's left of the buttondown shirt, you can add ruffles, sleeves, etc. to finish it off.