Turn the top inside out, working on the wrong side.
Lay the elastic along the pressed line and using a 'zig zag' stitch, begin by going backwards and forwards in one place.(Always leave a good inch to grab hold of at he beginning.)
Then as you really stretch the elastic start to sew along until you get to the end of the pressed line and zig zag backwards and forwards to finish.
This is a technique I use alot on tops and coats.
It gives shape to an otherwise shapeless top and adds detail to the back.
By adding the frill it transforms this top from a frumpy top to something more 'cute' and much more flattering.