How would someone grade a bouldering problem they've created? What would one have to look out for when grading? When bouldering, does a highball mean a more difficult grading?
Short answer: Climb lots of other routes in many different areas and have lots of other people climb your routes.
Let me get into why you opened a can of worms with your question:
Ratings for routes are almost always in a greater context both historically and in respect to their location. The people who created the Yosemite Decimal System for example had anticipated a ceiling of what is humanly doable, and when climbing rubber got better and better, harder and harder routes received the same rating (5.9) since that had been deemed the upper limit. This kept happening until the system was turned into an open ended one. Therefore, if you climb in the US and see a 5.9 that was established half a century ago, you are looking at a broad spectrum of difficulty. Some of those routes have received a grade-upgrade in the past decades.
Another phenomenon that occurs quite commonly is that locals perfect a certain technique, and to them a certain type of climb becomes much easier. This results in what is called "sand bag ratings," which means that outsiders might be in for a surprise if they only look at the rating in the guidebook. Yosemite national park can be an example for this, since the slick granite can be very different from what people are used to, and because slabs without much protection are quite common, especially in Tuolumne Meadows. Outsiders often need a couple of days to adjust before they climb at their grade.
This brings us to routes that require one very specific technique. If you don't know how to do a knee bar, a 5.11 climb might be 5.14 for you. Sometimes even years after a route was established someone finds a sequence that can downgrade a climb even by several grades. Technique (or lack thereof) also is a major reason why rating a climb is much more variable when done by beginning climbers.
Sometimes ethics play a role in the difficulty of a climb. Some routes are much easier if climbed with a knee pad, or with tape gloves.
Now enter the difference between gym and outdoor ratings. In my local gym I can climb one full number grade higher (YDS) than what I would be able to climb outside.
To make the concept of rating even more confusing, add different rating systems between climbing communities. The English for example take into consideration the difficulty of protection and the amount of exposure when they rate a route. Under the Yosemite Decimal System, a route has the difficulty of the hardest individual move, no matter what the rest of the climbing is like. The V-grades for bouldering is an even stranger animal, since the routes are so short.
Rating climbs occurs through a highly subjective consensus process, influenced by a multitude of factors, many of which I probably forgot to mention here. I hope I confused you enough that you forget all about it and instead just go out and climb.