NI3 isn't hard to make, there are only two ingredients and I also made it in chemistry class by pouring ammonia on the crystals which we left on filter paper. The real 'trick' is not to use that much iodine crystal so you don't get a huge bang. You want to stay under 0.1g of crystal.

There's precious little you can do wrong with the synthesis other than using too much iodine to make too much of it. It won't become particularly unstable until it dries a bit, but after that, yes, it may just go off on its own.