Take your fake heart (I found mine at Party City for $10) and cut a hole about an inch in diameter into the bottom of it. Don;t worry about neatness -- this will be underneath and not visible. This is where the main water line will feed through.
Next, drill holes into the two heart valves where your tubing will stick out of for the blood to squirt out. I say "drill" but I used a pair of scissors. This is all dependent on the material and thickness of your heart.
Now, place the two split tubes and the T into the large hole in the bottom of the heart and feed the two end tubes through the two holes in the heart "valves." This might be tricky, so don't worry about making a bigger hole if you need to. I found sticking something through the outside of the valve hole made it easy to fish the tubing through. After you pull the tubing through, push it back so it isn't very noticeable.