It's simple really, just get someone to pay you to take pictures. And come back again.
IMHO, a "Professional Photographer" has a complete understanding of exposure. I don't mean getting a "properly" exposed image, that's a given of course. I mean knowing how to use Depth of Field, Shutter Speed and other factors to produce a specific image that you want, to convey an idea, feeling, emotion, setting, etc. You should know the standard tehcniques for creating a specified image.
You should have an extensive knowledge of lighting. In the studio, if I say: " Give me 70 on the key, 20 on the fills and an orange gell on the snoot. Then back the key and bump the fills for a few Rembrandts." You should know how to use a light meter and where/how to place the lighting to achieve these recipies.
You should have a deep depth of pose, the classical poses, modern modeling and arrangement. Even if you specialize, you should still have a working knowledge of Portraiture, Landscape, Sports, Modeling, Commercial, Product, Photojournalism, Weddings, etc. You should be able to functionally assist any photographer in any given situation, at a level where your "2nd Shooter" images are marketable.
You should be completely comfortable working in a studio. If I say: "Roll out a 7' mottled seamless and a 6" Apple"...I expect it to be ready. "We're gonna shoot straight to Photoshop"...you need to have that setup and working. A functional working knowledge of Photoshop and Lightroom is expected. You don't need to be an expert, we hire those for that type of shoot.
You need to know your camera. And I mean KNOW your camera. Every button, dial, switch and setting. Why you use them and why you wouldn't use them. Have your camera properly setup at EVERY shoot. Know how to setup your camera for a shoot and what lens you will use and why. (why is it better to use an 85mm rather than a 28mm for Portraiture? Why is a 105mm even better? What is the advantage using a 200mm zoomed in tight rather than set at 70mm and being closer?) And please don't show up with less than a full frame. (although, a D500 with a 500mm lens is an incredible wildlife rig)
There are many, many things and I've only scratched the surface. If you set your camera to "M" and leave it there, you're probably on your way. (If you can sell those images)