I didn't have a large box but I had a bunch of small boxes. So I was able to do half of the heart and then I had to trace it on another piece of cardboard and cut it out. I made sure to have the middle overlap a bit, but this meant I had to cut away the excess at the bottom. Also, since this is going to be a box I needed a front and a back so I ended up cutting out 4 heart halves. I used scissors to cut the cardboard but you can also use a box-cutter and cut it out over a cutting board.
(I may not take great pictures but now and then I take a lot so hopefully it's easy to understand)
When I went to tape the back on I realized somehow it wasn't the same size, luckily it was too big so I just cut off the excess.