Flavor art's flavorings are pretty concentrated. I would start on the low side, personally.
I really can't tell you what to mix and in what percentages. All tongues differ. If you are coming from commercial juice, I'd guess an overall percentage of about 8--10%, and I'm still on the high side (for me, but I've been at DIY awhile, palates change.)
Choose a "dominant" flavor would be my advice, and make that your highest percentage. So pick a flavor that appeals. Then, start considering what you feel "may" add to your experience, and complement it. If you want a strawberry well, you're going to have to have the cream along with it. The good news is, that's not a disgusting flavor profile to many folks. Okay, so then what might complement that? A peach? A different fruit? Are you looking for any sour notes? Do you think you will need sweetener? Etc.
I started out the slow and laborious way. It's how I learned to cook, which is why I chose it. I started with small tester mixes of ONE flavor alone. I tasted it, so I could learn its "steep" time, and then I would combine single flavors in my tank. Not the ultimate BEST way to do it, but it gave me a sense of what I liked "with what."
If you have any actual "one shots" do a tester with those. The good news is one shots don't typically need much steeping if at all and it gives you something vapable while you are learning your other flavors.
Recommended percentages are "fine" to start with. You may find you need more flavor, but starting low, I can promise you, it's easier to add a bit more flavor than go down the dilution trail. Which I have done.
I also tried to fix every "failed" mix I encountered. It taught me a lot, even if my flavor remained, ultimately, a failure. I also, once I got a basic sense of flavorings, and how they combine, found my skills kind of "built" on themselves. If I have a flavor I know little to nothing about, I may still do a tester mix, because I need to "understand" how it tastes-- to me-- before I can have any success with it in a mix.
It's laborious, time intensive and hard at first, but as time has passed, I'm so glad I approached flavoring this way. I have never used a recipe (although I've READ a bunch) that wasn't self created, and I'm happy to say that. I'm no expert on flavorings, necessarily, and there are tons of folks who will be willing to help you out.
I mention my methods, not to say you HAVE to approach it that way, but you are sort of asking a question that can only be answered in generalities.
So my generalities: underflavoring is easier to cope with than overflavoring.
Small batches to start won't "waste" so many ingredients.
If your mix is a fail, try to fix it. You have nothing to lose, and plenty to gain.
Understand flavor blocks and how they interact
Write down everything.
Good luck!
Anna