Decided to make a second comment since I was about to run out of space in that one:
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My tips:

• Don't rely on using "Undo" (CTRL+Z on PC; the icon in the bottom left of the screen), becauase a LOT of the things you do in game are not "logged" like they should be and using Undo actually makes you permanently lose progress. I wish they'd address that, but... it's been ages. Use "Save" instead (CTRL+S). In the learning stages, it's fine to use, but once you start working with Text Labels and Override menu, that's when you want to avoid using it very often.
• After you place a Primary Wing, select it, then open up the Override menu (bottom right, next to Play), and where it says Wing-3, change it to Wing-2. That makes it non-flexible (aka 'strong') like the "Structural Wing" but able to have control surfaces like the Primary Wing. If you're making a build that the main wing is made up of Primary Wing and Structural Wing parts, that flexibility is a nightmare, (Confusing I know, but trust me lol)
• When it comes to wings, you want the big wings that make the plane fly to either have a "Semi-Symmetric" or "Flat Bottom" airfoil. For the tail fins, you want them to be "Symmetrical". Try to avoid using Symmetrical except for the tail fins (or "canards", which are on the nose).
• A crude rule of thumb for the "Center of" Thrust, Mass, Lift (aka CoT-yellow, CoM-red, CoL-blue for spheres in the designer): Keep the CoM between the CoT and CoL; but closer to the CoL. There are exceptions, but that will get you started. When in doubt, look at how other people have balanced that out. (If theirs strays drastically from that, they might be employing Funky Trees magic)
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  Good luck, and may the Force (of Lift overcoming Gravity) be with you.

Everything has been summed up pretty well. My suggestion is, if you're not already a coder, to worry about learning or creating complex Funky Tree controlled vehicles LAST.
You can manage to accomplish a lot of stuff simply by knowing the "IF ? THAN : ELSE" statement.
Granted, you need to know the various different input names/values in order to make that statement "work", but you'd be surprised how easy things are with just that lol
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Seriously, I can sort of read code snippets enough to follow along, but I'm a certifiable Coding Moron at the end of the day. Just knowing that single thing has let me accomplish some useful stuff (sadly I have no on-hand example for ya, wish I did).
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Instead, focus on how to use the game interface, and the "quirks" of the system (there are a LOT of quirks unfortunately). Then familiarize yourself with the parts, their individual good and bad attributes. Next work on getting a handle on creating a simple, but functional plane -- by that I mean that it flies well with minimal effort, and doesn't want to enter into a death-spiral when maneuvering (in other words: the fine balancing act between Center of Lift, Center of Mass, and Center of Thrust).
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Once you have it so you know those "basic" things involved to make a functional plane, learn how to manipulate the Fuselage part to make it look interesting ways, so that you can use it to create the curves your need.
Making "complex" looking builds will bring the challenge of your planes now not wanting to fly right, which will lead you to "XML Modding"; also know as "using the Override menu". As you'll need to turn off or decrease the amount of Drag parts make (as the game is nowhere near perfect in modeling airflow once you start making complex stuff), or lower the mass of others, even make others not physically interact with your plane so that they work as designed (ie landing gear).
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By THEN you'll be at a point where you'll want to learn the Funky Trees coding. (It's called "Funky Trees" because it's a language special to Simple PLanes which when created, was used to make the Palm Trees react as you got closer, which looks rather.... funky in their behavior)

I'd say it just takes time to get to know all the little things about the game and get very familiar with it. Also look into Funky Trees functions, it's helpful.

In my experience, just like everything else in life, it takes time before you can build creations like the top creators on this website. My number one tip is to just start making something without creating a specific expectation in your head; use other people their builds as inspiration, not as a benchmark.

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The easiest way is to download other users crafts and disassemble it and see how it's made (it's how I did it before I made this account)
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Now second way is to use blueprints blueprints on all side so that all corners are correct
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Or you could ask a very good builder to help teach you how to build on Discord
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You practice by looking at other builds and and try to copy it you then learn when using blueprints how to make correct shapes and corners
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Now I would suggest to do the first, second & fourth one because a lot of high ranked users are busy by building something new or just busy in irl etc so I would suggest one, two & four

Just get good

1) Just build what you are interested in (I am also a big fan of the Viggen)
2) Want to improve: Wanting to build better will naturally guide you to seek out more information
3) Don’t reinvent the wheel: Download other’s crafts if there is something you are interested in (a HUD, or custom landing gear for example) and see how it works
4) There is no “proper” way to play this game, just develop your own style and learn as you go