As dougolas said, make sure the ammonia you have, has no surfactants or any other additives. It will likely be a 10% ammonia solution with water.
Test your water to see if your substrate has leached any ammonia - if so, and you have a reading of between 2 and 4 ppms, that should be good to get things started.
Test every couple of days, and should it test out at 1 ppm or less, then add some ammonia - try 2 or 3 teaspoons of the 10% ammonia solution which should be good for your 40 gal to test out @ around 3-4 ppms. Keep adding a couple of tspns of ammonia every time the test shows the ammonia level in the tank has dropped to 1 ppm.
It will very likely take from 4 to 6 weeks to go through the entire cycle, during which nitrites will have peaked and reduced, and your water has developed a reading for nitrates.
When your tank tests 0 for both ammonia & nitrites within 12 hours or so of adding the last batch of ammonia - you'll be good to go. Just do a good-sized water change at that point to reduce the nitrates level.