Mustard seed plus water! You can use ground mustard seeds or whole seeds - if whole, smash them after they are soaked. Use cold water to make very hot mustard, warmer water to make milder mustard. Use enough water just to make a paste. Let it set for 15 minutes before mixing it with anything else. That initial soaking is what sets the heat for your concoction.
The heat fades, so add a little vinegar or lemon juice if you want to keep it in the refrigerator, and that will hold the amount of heat in the mixture. It's the process, not the ingredients, that determine how mustard holds its flavor.
Different brands of commercial prepared mustard have different aromatic spices added, so if you want to imitate a commercial brand, use the corresponding spices.
Also, try collecting the mustard pods before they are mature or before they are dry and see if you like the taste of immature or mature "green" mustard seeds.
