First make a ruffle out of your narrow lace. There are many ways to do this...
I use the longest stitch length on my machine and sew all the middle. Pull the bottom thread to create a ruffle.
Pin your narrow ruffle to the middle of the wide lace.
Sew the ruffle to the lace. Find the middle of your tank and mark it so you know where to attach the lace. (I did this by folding the tank in half and matching up the straps.) Pin the lace to the middle of your tank.