Divide the weight of the airplane, in ounces, into the square footage of the wing, (stab and elevator should also be considered to be more precise), as they are lifting surfaces at work when in flight, the area of the wing should be in square feet.
For example:
I'm not going to count the tail areas surface.
Wingspan: 72" (inches long) x 10" (inches wide)= 720 sq. inches
720" divided into 144 (one sq. foot, 12"x12"= 144") = 5 (sq. feet)

Weight: 10 lbs x 16 (there are 16 oz in a lb.) = 160 oz.

Wingloading: 160 oz divided by 5 = 32 oz per sq. foot

If you also take into consideration the stab and elevator, the wingloading will be less, of course.