How do they do it?

[Siggypoo72]

Out of so many airfoil designs out there, how does a budding designer discern which airfoil best suits their homebuilt concept? I've asked the same question over at RC Groups, as I've been a scratch designer and builder since childhood. But I've always used the same methods for figuring out the airfoil that works for my RC models; trial and error, the TLAR Method, and SWAG Theory.
Having designed and flown many an RC plane of my own design, in many sizes and formats, I've been inspired by some projects on YouTube to try a man-carrying plane of a particular design of mine; an upright gull-wing reflexed airfoil with a single or dual RC turbine in a centre pod, or bulge. Wing span is 14' and wing chord is 3'.
The pilot lies prone on top and controls flight by shifting weight. Made of foam core with a fibreglass shell.
Basically a rigid powered hang glider with the pilot on top.
The airfoil I'm leaning towards, being low drag and high lift, has been the Roncz Marske 7, and it's worked nicely so far for the RC models of the same design.
The (perceived) benefit of using RC turbines is that a dummy human-weight could be placed (via servos) atop the airframe for flight testing, and the aircraft flown from the ground.
But, with hopes of having this homebuilt certified someday, I don't want to play the design by ear. I want real-world guides to help me narrow down which airfoil would best suit my needs.
I suppose each new aircraft design has risk because, ultimately, some poor schmuck has to take it up and in, right?
I want to know the best airfoil for my needs prior to wheels up, and I've no real-world aviation design education.
My ultimate question is, how do designers choose the right airfoil?
Any help, advice, or even flaming is very welcome.

Arthur