Of all the specialized woodworking names, donkey's ear is definitely my favorite, although until now I haven't felt the need for one. That changed recently because I wanted to make a box 3" high with precise mitered corners.
I built one very similar to what you commonly see, basically just a 45 degree ramp with a fence:
Not hard to see why it's called a donkey's ear. It is made to be used with my shooting board:
For now, I just clamp it in place. It is made from scrap pieces of baltic birch plywood, glued and nailed together in an hour or so. Baltic birch is ideal for this because it is stable and flat, but there was a problem for me. Since I sold my table saw and am unwilling to use my handsaws on plywood, I could only use my old chopsaw to cut out the pieces. Even after several tries, I just couldn't get the pieces absolutely accurate, which caused me to ponder what to do for several days. I'm not about to plane plywood to true it up. I decided that I would make mine as accurate as I could and then resort to my usual: shims made from playing cards with the possibility of adding adjusting feet later. When I got it done, I found that it was off about half a degree, enough to produce a very slight gap. I was about to shim it up when I realized what is likely obvious to the rest of you: my plane has a lateral adjuster! Slapping my forehead, I tried it and, sure enough, the result was quickly dead on. There is something immensely satisfying about producing a box with four perfectly mitered corners in just a few minutes.
There are some really beautiful donkey's ears offered for sale that are made to extreme tolerances. I'm not sure it matters. As long as you can get a consistent angle across your donkey's ear that is close to 45 degrees, you can use your lateral adjuster to get a perfect fit. So, if your donkey's ear is too low, don't worry. If you are so inclined, however, this looks like a nice one.
Like a shooting board, a donkey's ear is a handsawyer's enabler: you can go from close to right on without a lot of fuss.