How can I keep my waffle iron relatively clean when making waffles?

Replace your ladle with an ice cream scoop.

You will be able to control the amount of dough for each waffle much easier. For my waffle iron and scoop I use two scoops.

An additional advantage over a ladle, is that the dough releases much easier from a scoop, which reduces the probability of making a mess.

Spray non-stick oil on the outside edges of the waffle iron, or spray some on a paper towel and wipe the iron's edges. Also, find a measuring cup that has the correct amount of batter for one waffle. Mine is a level 1/2 cup.

I fear that there isn't really a "correct" answer to this question: it's almost impossible to tell how big a waffle iron is (in terms of how much batter it holds) without just experimenting to see how much batter it holds. The amount you need will depend on the size of your waffle iron, whether it's round or rectangular, and how deep it is.

Similar to what other people have said, I figured it out by trial and error. I use an almost-full ladle (scraping the bottom of the ladle over the lip of my mixing bowl to prevent drips) and pour the batter to the middle of the pan. That's the perfect amount for my waffle iron.

I actually don't recommend spraying the outside of your waffle iron with non-stick spray/oil. In my experience, it makes things messier in the long term. The oil gets kind of cooked on because it's constantly being heated.

To make cleaning easier, just clean it as soon as you're done. If your waffle iron is hot enough, any drips on the outer part may have cooked to a crisp, meaning you can just scrape them off with a spatula or knife and be more or less done. The other thing I do that works on mine is wipe it down with a sponge or damp paper towel while it's still warm (just be careful not to burn your fingers, and don't use a plastic scrubby sponge that might melt on contact). If there's a lot of grease on the outside of the pan, a drop of dishwashing soap on the damp paper towel or sponge should help.

I usually find the worst mess is from the drips on the way to the waffle iron. Try putting batter in something with a "pouring" spout (large measuring cup or bowl with spout or even a pitcher) and pour batter in. Pour into center of waffle iron and don't let it get all the way to the edges - that will be too much. Let it get about halfway to each edge from the center, stop adding batter and close.

Apart from soegaard's answer, I'd say, don't even try! As the batter outside the iron becomes hot it will become a waffle, then you can just cut it on the outside and be done with it.

I don't know what size or kind of waffle iron you have, but the manual for my Black and Decker Belgian Waffle maker says to use 2/3 cup of batter and pour it right in the middle of the iron. It'll pretty much spread out on its own but you can use a rubber spatula. There are pancake cup/dispensers you can buy to pour the batter. Some have long spouts. Others have a trigger and dispense from a hole in the bottom of the measuring cup.

It also says to never use non-stick spray on the iron. You season the iron with the first use by using a brush to paint the plates with vegetable oil. Heat for 5 minutes, then let cool completely down. First waffle you make, throw away because it will be greasy with oil. After that, your waffles will be fine. It's already been seasoned so no need to paint more oil on it.

Not sure about the first part, feels like it is part of making waffles, failing the first or two, or three, or four with a new recipe.

But the second part is easy, while the thing is hot, leave it on a little longer so everything burns to a crisp, wipe the inside with oil. when it cools, wipe away any extra oil that is left, and spray the outside with a bit of dish soap + water if necessary. I find that much better than washing if you use the iron often, next time it sticks less.