I bought 30% Shrink Magic from Logans nearly 2 years ago...so it was high time to try it out!! (other people sell similar products with different names.) I put it against the wrong side of my fabric and then sewed along the lines for the right side. Here it is from the wrong side.
With the product uppermost I steamed it from about 1/4" away - here is the start of the shrinking.Here is what the shrinking looks like from the top.
And after intense steaming, this is the final result - the centre has been untreated, that's why it's popping up like that!