Popovers are tricky, but they can actually pop in something as simple as a non-stick muffin tin, so don't despair! First, preheat the oven to 450 Fahrenheit. You will bake at this temp for 20 min and then you will turn the oven down to 350 for about 20 minutes more (it depends on how large your popovers are). Keep the oven closed during baking, unless you must rearrange them once because your oven bakes unevenly. If so, do it later in the baking time. When mixing up the batter, do not use too much flour. If you need to, weigh it. My experience is that too much flour means they don't pop well. Also, I don't beat the batter to death. I use the whisk gently. There are still lumps when I am done. I usually let the batter rest a bit before baking. Uh, maybe 15 min or more?
I use 2 large eggs, slightly beaten or whisked. Then I add 1 cup milk (2 percent will pop). The dry ingredients are just 1 cup flour and half a teaspoon of salt. I add no butter, except that I butter the cups or tins. I don't pre-heat my cups, but some people do. If you do, maybe use vegetable oil and not butter. Don't overfill the cups. If you use six ounce dessert pyrex cups (mine have thin walls), it will make six popovers. If you use standard muffin tins, it will make about 10 (I fill the outside cups and leave the center of the tin empty and have a tiny bit of batter left over). Try to spread out your cups in the oven. I haven't used the cups in awhile because I broke one, and don't have enough anymore. When I use the muffin tin, the popovers often tip over out of the tins when they pop. Don't worry, they fall on the pan, and not onto the oven floor. The ones in the muffin tins aren't as custardy or eggy inside, though. They have more crispy surface area. This could be considered good or bad, depending.