Heart is the gateway offal. First, it's pure muscle not some biological filter or squishy part. Second, it is very clean tasting. Most will think they are eating a lean sirloin cut if they didn't know it was heart.
Trimming a heart of fat and the connective tissue is the first step for just about any preparation. It's not difficult but it does require a bit care and patience.
Begin by cutting the top off of the heart where all of the arteries and veins enter.
Then trim the outer membrane off the heart using a small, sharp knife. Shave off a little bit at a time while not cutting too deeply into the muscle.
Once the exterior is pretty clean, cut into the heart to begin opening it up "butterfly" style or like a book. Basically you're just trying to open up the chambers so you can trim the connective tissue inside. Trim the stringy, connective tissue inside the chambers using the same technique as shaving the outside. Continue opening up the heart as you trim so it will eventually "unroll" and lay relatively flat or in pieces.
Once the heart is trimmed you will have pieces of very clean looking, lean muscle. Now you're ready for the next steps in whatever recipe you choose.