Before you begin, look over the steps and photo tutorial for a sparkleball. The materials and method are much the same.
With a hot soldering iron, poke a 1/2" hole in the bottom of all 50 cups. Divide cups into two groups of 25.
Arrange 13 cups in a circle. How these cups are joined together will turn them into a heart-shape, so the next steps are crucial.
At left is a diagram I made of the first layer, with arrows showing where cups will be joined. The red dots are where lights will go. To see a larger diagram, go to my sparkleball website. (Google sparkleball.)
Be patient and let each solder joint cool completely before moving on.
Join the 2 top cups, #1 and #2 at top rims only. This forms the top of the heart. Join the next 6 cups-- adding 3 on each side-- at cup bottoms only.
Cups #9 and #10 are different. They're joined top and bottom to #7 and #8. Join #11 and #12 at tops only. The final cup, #13, is the heart's point. It's joined at its bottom to #11 and #12.
The first layer should now resemble a heart. The joints must be strong enough to keep the shape as you add the 2nd layer.
The 2nd layer has 9 cups. Add #14 at the top, first. Nestle it into the space between #1 and #2. It may almost point upward. This forms the heart's center dip. Now place the other 8 cups around the 1st layer, continuing the heart shape. These 9 cups probably won't touch each other but will attach to the layer below.
Fit the last 3 cups into the open space before you solder them. When you're happy with the fit, then join them in. It should now look like a fat, smashed heart.
Thread 50 of the red fairy lights into the this half. Each cup will have 2 lights inserted into the one hole. Start threading at Cup #13, the bottom one.
Build the second half exactly like the first. NOTE that on a heart-shaped sparkleball, you must begin and finish threading lights on Half #2 exactly as you did Half #1-- at Cup #13. (This is different than a sparkleball, where it doesn't matter since it's round, all round.) You want to be able to join the heart halves perfectly, and the lights must match up.
Join the two halves securely, taking great care not to burn the wires or lights.
The finished sparkleheart will be 2-dimensional (not 3 like a sparkleball) and looks best hung against something so you can really see the heart shape. It doesn't work as well if it's free-hanging.
Happy St Val's!