What is a foolproof way to season a cast-iron skillet?!

What is a foolproof way to season a cast-iron skillet?

I have been using just regular liquid vegetable oil- should I be using shortening?
I tried lard but the skillet stayed greasy.
If I put the pan in the oven upside down, it doesn't season evenly. I've looked online but so far whatever I've tried from there hasn't worked.

It's not a one-time process. You're doing just fine if you're lightly coating it with, say, Crisco and setting it in the oven upside down. Bake it at 350 degrees for an hour and then let it stay in there until it reaches room temperature. Take it out, wipe it down inside and out with a dish towel - not paper towels because they tend to shred and leave tiny little lumps of paper sticking to the iron.
But like I said, it isn't a one-time process. You will find that you need to re-season it periodically - that fabled no-stick surface all the books talk about takes a couple years of continued use to build up. Frankly, I don't wait that long.
What I do is, I buy pre-seasoned cast iron when I can, and use the technique described above for un-seasoned iron. After seasoning, I keep on using the items frequently, and after each use, wipe it completely clean and then rub a light coating of vegitable oil on the inside. I store them in a dry place, with all skillet and iron lids on their sides.
And I learned that, if you use lard, bacon grease or other unprocessed fat, and don't re-use the item fairly soon, the fatty coating can turn rancid. I opened the cupboard where I store some of my iron cookware the other day and noticed that, in the back, one of my older iron pots had greenish mold growing on it, and boy, it stunk, too! I had to take it out, wash it good with soap and water, and then competely re-season it. When it went back in the cupboard, it had a thin coat of vegitable oil on it. And from now on, it will come out once every so often and get used so that does not happen again.
Ya know, that's why cast iron has fallen into such dis-favor among cooks who've gotten used to the much lighter and easier to care for aliminum and steel alloy cookware. But to me, tradition matters very, very much - and I simply can cook tastier, healthier meals in my cast iron.
I particularly like to cook in iron in the great outdoors - nothing like a big pot of beef stew, chili and beans or chicken and dumplings in a huge cast iron, three-legged camp oven. My pards in our Civil War re-enacting unit love it when I bring along the cast iron 'cuz they KNOW they gonna eat good after the re-created battle in the afternoon! (We're the 12th United States Infantry, Army of the Potomac)