Rinse the whole heart, I get these from a local butcher so they are always fresh. Tesco has these as well. This is a very easy procedure and does not look like a heart when you are done.

Cut the heart in half lengthwise, now rinse the insides to remove any blood still left in there.

First thing is to trim away part of the top section, there is a good amount of fat and the main arteries, veins, and valves. Then remove the stringy parts. The cleaned portions are on the right, the scrap to be discarded is on the left.

And the cleaned pieces turned over. Just repeat the steps for as many hearts are you are going to prep.