It's a pretty simple process.
First, clean the heart well under running water. Make sure the water gets into all chambers of the heart and any blood clots get flushed out.
Then, trim off the top of the heart, where the aortas come out. They are rather tough and not nice to have in your mouth. Also trim off any fat that might be on the heart. Some deer have fat, others don't. In the two pictures below, you can see a little fat (the white stuff) around the top of the heart.
Next, put the heart into a sauce pot that is big enough to cover the heart with water. Now cover the heart with water ;-)
Put the pot on the stove and heat to boiling.
Once it comes to a boil, put a lid on the pot and turn the heat down to a simmer. Simmer for 20-30 minutes until the heart is cooked completely through. (When it's done, it will be uniformly brown all the way through if you slice it).
Remove the heart from the pot and put on a cutting board. If you have dogs or cats, don't throw out the liquid that remains in the pot. I save it and add it to their food for extra protein and a treat. You could even use it yourself as a soup stock, but we aren't that gung-ho here. Mainly because the liquid doesn't smell all that appetizing.
Allow the heart to cool enough that you can handle it without burning your fingers. With a sharp knife, cut into 1/4" slices (slicing 'around', not the 'long way' ie. top to bottom of heart). You do start at the top and work your way to the bottom, though. Hope that's not too confusing. As you are slicing, you may or may not have to do some trimming on the inside, where the valves are.
Place the slices into a deep dish or a bowl. Add sliced onions and 2 Tbsp pickling spices.
Over this, pour enough water/vinegar solution (50/50) to cover the heart and onions.
Refrigerate overnight before eating (if you can wait that long!)
As a side note, a deer heart (or other large mammal heart like a hog or steer) makes a great hands-on science lesson for a kid (or an adult!). When washing out the heart, if you squeeze rhythmically, you can actually imitate the pumping action of the heart in a living animal, with the water moving in and out of the chambers of the heart. Try it! Let the water run in one of the aortas but not the other; squeeze the heart to 'pump' it, and see the water come out the other side.