The concept is simple - wrap and attach a plastic bag around the stem of a very young apple. The bag needs to be big enough to have room for the apple to grow.
It's not sexy but it works.
Using this method is up front labor cost only but, in our case, the bags are free (with a paid subscription to the local newspapers). Commercially, there exists a product that does the same exact thing with the same result and takes less time, but it costs money and needs to be applied regularly: kaolin clay, aka Surround.