On each wheel with drum brakes there are two shoes that are pressed against the drum when the brake pedal is pressed. The object of adjusting the brakes is to get the gap between the shoes and the drum such that there is the minimum distance for the shoe to travel without the shoe rubbing against the drum when the brakes are not in use.
The drum is the big heavy round thing that you see when you are changing a tire. The shoes press against the inside of this drum. The shoes are on the front and back inside sides of the drum and pivot around their bottom-most points, with the top-most points being pushed outwards by the action of the slave cylinder mounted between the front and back shoes.
Adjustments are made by moving the bottom pivoting point closer or further away from the drum. It is important to keep this adjustment correct or the shoes will wear unevenly top to bottom, limiting the life of the shoe.
The bottom of the shoe is attached to an adjuster that can be turned by means of a star to move the shoe relative to the drum. The adjusting stars (one for either shoe) will be visible through either a hole in the drum (you'll need to turn the drum to find them) or through two holes in the backing plate, depending on the year of your car.
The adjusting stars can be turned by levering them with a screwdriver. Turning them clockwise tightens the shoes to the drum. The object is to turn them until the shoes bind against the drums and can't be wound in any more, and then turn them back three notches on the star, at which point the wheel should turn freely.
There are variations on this method.
It is possible to work out which way to turn the adjusting stars if you imagine what is happening inside.
Adjust one shoe, then the other. Then press the brake pedal a couple of times and adjust them again. This ensures that everything is centered properly inside the drum.
If you are adjusting the rear wheels, you'll need to adjust the handbrake after adjusting the brakes.