How to make a remote controlled airplane with balsa wood and card paper?

Try getting a plan for the plane, which you want to construct, online or something or buy a kit and then construct it based on the blue print. I feel you should buy a kit and then make it using the wood given in the kit, as that will be the best. If you are in US or know someone in US, try getting a kit from US or else get a plan, purchase wood separately and make it. A kit includes only the plan and wood, nuts, bolts and stuff and that's it. You have to do each and everything on your own. Only in an ARF (almost ready to fly) kit, everything will be almost ready.

If you do not want a kit and want to make each and everything by yourself, go through this article:

Aeromodeling for beginners

If you do not even need balsa wood, you can start with card paper planes, which are very easily available and cheap, and any one can afford to waste it if the design is not right, then graduate to balsa if you know the basics of flight. Soon you can upgrade to battery﷓operated models or petrol ones once you are familiar with their working well enough.

Everyone wants to just rush to making RC models without even knowing how and what makes a plane fly, and how the control surfaces work during the flight.

How to select an airfoil?

Selecting an airfoil depends on the type of model that you want to make, like if want to make a glider then a high l/d is required, and if an aerobatic model, then symmetrical is preferred. Similarly, if you want to make a high-speed flyer, then low drag airfoil can be selected. You can rely on the data given by designfoil software to have a right airfoil for your model.

The choice of location of CG is based on the type of model, i.e., stable or unstable, you want to make. For a stable model, you take CG ahead of AC and for a aerodynamically unstable model, you take CG behind AC.

There are many other factors, which decide the designing and performance of an aircraft.

AC is the aerodynamic center of the wing, which is the point about which the pitching moment is constant, but I don't think such complex calculations are required for making a model. Only real aircraft manufacturers use such terms since the perfection factor for them is much higher. We can make a good enough model by just ensuring that the entire wing is of an uniform airfoil section which is selected beforehand, just take care on the balancing, as using too much correction from the elevators for balancing might ruin the performance of your model.