Slice the very top off the heart, which is mostly veins. You can grind this up for burger, or feed it to your pets, or toss it in the stock pot.
Trim away the fat from around the top of the heart. This is very hard and waxy.
Use your knife to open up the heart's chambers along their natural seams. You'll do this twice, and the end result opens the heart up like a scroll or book.
Carefully slice away the veiny bits. Again, these can go into the grinder, or stockpot, or your pet's bowl.
One end of the heart will be twice as thick as the rest. I slice that in half lengthwise so it's the same width as the others. Now you're ready to go!