Really encapsulate what your vision is for your build. Choose an architectural building design. Like futuristic, modern, etc. load up on fuselage parts, some of my builds only contain fuselage parts. Use a skybox for the screenshots and turn the reflections on high. If your building a plane but you want a lot of parts for detail. Use xml mod to remove weight and drag on parts before you start building.

I try to unify my design around a certain concept or function. When building fictional, ask yourself questions such as “how can this shape help my plane get its job done?”, or “how does my [insert concept here] fit within this plane design?”. Start with something that seems pretty good, and change the design as you go!

Thank you everyone

You just gotta look at the plane and shapes that it’s made of, and you’ve gotta copy them. The more you can be bothered to do and the more precise you can be bothered to do the better I guess lol. And with fictional planes, you just gotta look at the shapes and techniques on real aircraft and reflect them in your own design.

I’m not much of an advanced builder, but I’ve tried before to some effect, so here’s what I’ve got. Do what people have already said: shape, maneuverability based on design, references. In my experience in making smaller details, I find it’s best to do major things first, then add smaller stuff. My most popular creation is a ship with some smaller detail, but nothing way too detailed. My only plane with fully functioning stick movement has basically no other detail besides that, but it’s enough. Basically, get the shape first, then work on smaller details one step at a time. If it looks good with a couple minor details, don’t over complicate. Sorry if this didn’t help at all.

I'm no advanced builder, but heres an upvote to spread the word

Here is what I do:
I ask myself these questions every time:
Does is look and fly exactly like it does in real life/how I want it to. And if not, why not. Fix it.

After you get the shape and details right, also consider flight characteristics. If it's a stuntplane or fighter, make it maneuverable. If it's a large airliner or cargo, make it fly stable with slow acceleration. Custom control surface and correctly modeled aerofoil is also important. Custom engine and casing can give your build a bit of realism. Overall, if you're building something, everything counts as importance. Shape, scale, details, flight characteristics, etc.
If you're on PC, I suggest using mods to enhance your build. Designer mods like Finetuner, Overload and Designersuite can make building advanced and detailed builds easier and faster.

Going off of what everyone else is saying, You want to get the shape of the build right before detailing. Detailing should be like an extra treat not the main focus here.

Thank you @CoolPeach @Armyguy1534

As randomusername said, the shape of the plane is probably more important than any details, you wanna focus on that first.

No problem! @Mercyaircraftcorporation

Give it lift

Thank you @Strikefighter04 @randomusername

Look at a picture of what you are trying to make. For example, I'm going to start on an Anti-Riot Vehicle, so I am basing if off of this. Use your resources.

Thank you! @CrashFighter05

Add in some form of minor detail. Lettering, fake buttons, stuff like that