This is a complicated question that I am working diligently on a video to respond to better.

A few things:

1. Many concentrates "share" the same individual aroma volatile compounds. If you mix two concentrates that share a common compound, that compound's flavor base will be more pronounced possibly than some of the unshared compounds.
2. Aroma compounds can change flavor massively when mixed. If you have a compound that contains pentanol and another compound that contains acetic acid, those two combine to form a banana flavor. Oops. If you have one that contains pentanol and another that contains butyric acid, the ester from combining them tastes like apricot. Also oops.
3. As you add more concentrates, the risk of amplified, muted or changed flavor notes is multiplied. This is why it is not recommended that new DIYers play with too many flavors at once. I always recommend starting out with a simple 3-4 ingredient shake-and-vape recipe. Try it out and then make changes to it yourself -- lower one ingredient, raise another, see what happens.
4. Different compounders (the companies that make the concentrates that are sold by TFA, etc) mix at different solution levels. Some compounders create very dilute concentrates so that food customers don't abuse the concentrates. Those concentrates might be 5000 PPM of aroma compounds. Other vendors make much more aggressive concentrates which are 50,000 PPM of aroma compounds -- you have to use 10% as much as the weak diluted once or you get ageusia, dysgeusia, hypogeusia or anosmia (loss of taste, smell), or harshness, or soapy flavoring, or just overall a bad mix that sticks to your atty for days.
5. Over time, basic experience will bring you to understanding how different compounders create different solution percentages of aroma compounds so you have a better idea of where to start. FA is super potent, TFA is less potent, generally.

I haven't been doing this for even a year yet, and I feel I have a good grasp on what you're searching for.

The trick is: help others as much as you can. It's surprising how much this expands your own knowledge. Even when I only had 5 flavors that I had ever used, I still helped people when those flavors came up in a question. From there, people would reply to me, adding their own experience to the comment, or correct me, or completely agree with me. Either way it turned out, I was learning at an incredible pace. Most of my knowledge doesn't come from hours locked away in my mixing room, experimenting with flavors, but from everyone here sharing their own knowledge with me.

By far the quickest way to become comfortable with your mixes is to help others whenever possible. The knowledge gained is absolutely priceless.

I've been DiY'ing for a few months now but honestly I cannot say I have learned much.

I've learned that poking around the Internet will lead to a lot of other people's opinions, recipes, and reviews on flavors. I say I haven't learned much because I seem to be afraid (?) to dump a lot of money on certain flavors that I have no idea about. Although, my collection has grown to about 25-30 flavors but to date I only have two ADV's. Those are adapted from someone else's recipes, I just changed the %'s to fit my vape style. I tend to try for more flavor builds.

Any how, I mixed up 5 new recipes today and will wait on them a few days to test them. I pulled out all my flavors today, and almost had zero inspiration to figure out how to make something work. They all felt very clunky, and not imaginative enough to blend together. I feel I have decent building blocks, but no main structure. I also have a weird quirk about sweeteners. I Have had bad luck with EM as gunking up my coils way sooner than I would like to clean. I feel the cleaner the juice the better. I typically stay away from diacetyl as well and that makes a lot of people's recipies a little more difficult. So that being said it's a grind, just need to put some energy into it.

That is what I am trying to find. So , as usual abada's info helped, but OP I can relate. I just may be late to the party.

Edit :Sorry for the rant, this all seemed to come out.

I have been trying to frame this same question myself....

Personally, I fall back on my culinary experience to guide me about 70% of the way to a finished juice. (Keeping in mind that flavor concentrates almost never work in the same way the food does, but also sometimes ignoring that fact to keep from getting discouraged) The rest is a combination of trial-and-error, experimentation, inspiration, recipe research and happy accidents. The exact same process I use to create a dish. It ain't pretty, economical or easy but that's how I do it.

Start with simple mixes, like 2 or 3 flavors. Let it steep a few days, try it. If it's not exactly what you're looking for, tweak it a little bit. Rinse and repeat.

Most of my mixes start like a blind shot; "Hmmm these could taste good together." or "I'd like a blueberries and cream mix". I try it, sometimes the profile is good enough for me to make it a WIP, sometimes it's not and I move on to another. I usually have a few of these prototypes steeping at all times.

When I do get a good profile, it's trial and error. For example if you want a strawberry and cream, pick a strawberry flavor you like and try it with a single cream or two of them. Eventually you should have the knowledge of what most of your flavors taste like and react in a mix. For example, Pear tastes like pear, sure. But add 1% of it to an apple mix to help the apple come out more. Same goes with Lemon and fruits in general.

But to answer your question, in my case it's mostly trial and error.