I only take 10 days of travel per vacation (plus some recovery time at the end), but am lucky enough to be able to do this twice a year.
So, I start by looking at a limited area per trip, assuming I will get to other areas on other trips. In other words, I don't go to "France," but instead to the Riviera on one trip, Paris and Dijon and Lyon on another trip, Provence on another trip, and Stasbourg and Nancy and Paris on still another trip. Right away, cutting down what I plan to see makes planning easier.
I cast a wide net when looking for ideas. I read this Forum as well as other travel forums, get travel guidebooks out of the library, and watch videos. I don't just use Rick, helpful as he is, because his books are deliberately selective.
As I'm deciding where to go, I start investigating transit links (I don't drive, so I have to make sure trains and buses will work for my trip). I also start looking at possible arrival and departure airports. I always try to fly where I want to be. For instance, if you want to visit Scotland, fly to Edinburgh or Glasgow. Even if it's "cheaper" to fly to London (and it may not be) you waste a day at each end getting from London to Scotland, as well as it costing money. I'm lucky that, living in New York, I have a large selection of nonstop flights, but I'd still change planes to get where I want to start and end the trip. Of course, if I can arrange it to be near a nonstop flight, particularly on the way home, that's better.
When finalizing which days to spend where, I now make sure that I don't end up in a place on the wrong day. When I went to Nancy in France, I was there on a Monday and a Tuesday; guess which two days the main museum, which I really wanted to see, was closed? Now I check for such things, which helped me on my last two trips; I avoided being in museum cities on a Monday, which had inadvertently been part of my initial plan.
I do all of this before I actually book anything. I've seen too many posts of people who were locked into flights that didn't fit their desired itinerary, or had booked nonrefundable lodging and now it didn't work for their plans.
I've learned the hard way that, when it comes to deciding how long or short to spend in a place, there's no way to know for sure except in retrospect. You simply don't know how you will react to a place until you've been there. With new places, I try to look for day trip options, in case I feel the need to "escape." But even with several decades of travel, I mis-planned my trip to the Basque country, and had far too much time in one area, as well as not understanding how rural the area is (even most of the "cities" aren't that city-like). I know you're just starting to learn about planning, but just know that even us "old hands" still make mistakes!
Most important when planning is my travel mantra: On this trip, I will see what I see and miss everything else, and that's OK because it's all good. That's how I keep myself from going crazy with all the things I'm not seeing on a particular trip.