You need a sharp knife.
Start cutting at the thickest part of the breast where the two halves were joined. Make a cut along the inner edge of the breast bone, keeping the knife blade close to the bone. Pull the freed meat back from the bone and, keeping the knife edge close to the bone, make cuts along the line where the meat is still attached to the bone, working along to free more meat from the bone, pulling the freed meat back out of the way. Continue in this manner 'til done.
There is a breast tender on the under side of the breast, you may leave it attached or remove it with your fingers and save it for another dish.
"If you want to make an apple pie from scratch, you must first create the universe." -Carl Sagan