A single training plan is simply one step in a much longer journey for the purpose of endurance running. The true gains will come from stringing together multiple training plans in an effort to reach one's goals.
So you have to ask yourself, "Why did I sign up for this half marathon and what are my goals?" That would dictate what you plan to do over the course of the next year plus. Your goal is personal and there are no wrong goals. You could say-
I want to finish. I want to break 3:30/3:00/2:30/2:00/1:30/x time during my half marathon. I want to take pictures with every character and finish with time to spare. I want a dark Magic Kingdom Castle picture. I want to be in a fitness place where I can goof off with friends and have a good time on the course without a worry. I want to finish feeling good and enjoy my Disney vacation. Etc. All of these are valid goals. I gather from this:
https://www.disboards.com/threads/talk-to-me-about-marathon-weekend.3814231/
that your goal is to finish the half marathon and still be able to enjoy your family vacation without interruption. So, my response would be, the better fitness position you are in on the day of the half marathon, the more of a buffer you'll have from the balloon ladies cutoff. Thereby, putting you in a position where you don't have to necessarily try as hard on race day and still finish without worry. By putting in less effort on race day, you'll be less sore afterwards and able to enjoy your vacation without interruption. So let's say you were in either 2:30 or 3:15 HM shape (and I've got no idea where you actually are, but simply as an example). The 3:15 HM runner is going to have to put in a harder effort relative to their own fitness level in order to finish the race before the cutoff. They are likely to feel more sore post-race. The 2:30 fitness runner could also run the HM at 3:15 HM pace. They would feel pretty good after the race and probably have a low level of fatigue from the event.
So that's where we circle back to the beginning of the broad answer. If your goal is to put yourself in position on race day to be able to finish with minimal interruptions to a family vacation, then your training starts today (well not literally today, but you get the picture). You'd want to lay out a multi-training plan approach leading into race day to maximize the time you have between now and then. It's the big picture approach. Because if you're currently a 3:15 HM runner, maybe depending on how training goes and your potential, you could be a 2:30 HM runner in January 2022 after multiple good training plans.
As for the other part of the question pertaining on "how do you come up with a training plan?", my first advice would be "don't" (especially since it's your first). Don't come up with your own and rather use a plan you can find that best fits your life and what you've been doing in the past. For instance, let's say you've been doing 3 days per week and a max of 60 min per day. I wouldn't suggest picking up a plan that is 7 days a week, multiple double workouts, and maxes at 240 min in a day. It's an extreme example. But the point is, you want to find a plan that bridges where you are to potentially how much availability you have. If all you can work into your life schedule is 3 days per week, then choose a plan that is 3 days per week. If you can pull off 4 days per week, and you were previously doing 3 days per week, then find a 4 day a week plan. But try to choose something you think you can commit to nearly 100%. You don't want to choose a 5 day a week plan knowing you'll more than likely not be able to follow many of the time or day commitments. At the end of the day, the author of those plans wrote it that way for a reason. And the reason might be more complicated in reality than it seems on the surface. So making edits to the plan could be changing things that might make a difference on race day. There are plenty of plans you can find available online for free. The runDisney site has some Galloway ones and Hal Higdon is a common place for free plans as well. I urge you to read the instructions for either of those though because you'll get more out of their plans by doing so.
But let's say your life is not traditional. You can't commit to running consistently 3 days per week on a M, W, Sa schedule. Maybe some weeks you can do M, W, Sa and other weeks T, W, F, Su. So on and so forth. Maybe it's because you travel for work, or are a nurse with weird hours, or your kids have variable schedules that need to be worked around, or any other completely valid life reason. Then you're going to be best off either working with someone else, crowd sourcing with a place like the DIS, or as a last resort for a new runner writing your own. If you end up deciding you really really just want to write your own training plan, then I wrote this coming up on 3 years ago.
https://www.disboards.com/threads/how-i-write-a-custom-running-training-plan.3661946/
It goes step by step in the process of how I write custom training plans. It's written in a way that you yourself could write the plan for yourself by yourself. Admittedly, it's long and there are a lot of details. But when someone authors a training plan, these are the things they take into consideration. Above all else, I would focus on training at an appropriate pacing. That'll lead to the most enjoyable experience in the short term within a single training plan as well as the long term over the course of the next year or so. Hansons, McMillan, Daniels, and Galloway have calculators that give a range of paces that will be useful. None more useful than easy and long run paces. You may be shocked at how relatively slow the easy/long run paces are, but trust me "train slow to race fast" works.
https://lukehumphreyrunning.com/hmmcalculator/race_equivalency_calculator.phphttps://www.mcmillanrunning.com/http://www.jeffgalloway.com/training/magic-mile/https://runsmartproject.com/calculator/