You can do preacher curls in several different ways because it's that type of exercise that has quite a few variations.
First of all, all they have in common is the preacher bench that provides the required inclination to perform the curls.
The variations are about what you can use as weight:
I'll describe the most common here, which is the EZ bar preacher curls based on my experience, even though I've used the dumbbells too.
Quite less popular is the variation that uses the cables.
PRIMARY MUSCLE: Biceps
SECONDARY MUSCLES: Forearms
EQUIPMENT: Preacher bench, EZ bar
Curls on the preacher bench are extremely effective to work on the lower part of the biceps. The inclined pad support avoids the use of your shoulder, while your forearms are still used as secondary muscles.
INITIAL POSITION: Seated on a preacher bench, grab the EZ bar, shoulder width. Make sure that your elbows are supported on the pad and your armpits are well on the top edge to give you a stable position.
MOVEMENT: Curl the bar till your forearms touch your biceps, typically your forearms are vertical in this position of max contraction. Then slowly return to the initial position.
BREATHING: Exhale during the active phase of the exercise (when you raise the bar) and inhale during the negative phase.
TIPS and ERRORS: During the negative phase, when you lower the bar, don't stop too early, continue till your arms are extended. Many people stop before, but the exercise is not complete if you do that. You need to stretch your biceps during the negative phase.
It's very important to keep the tension on the muscles, so don't even raise the bar too high just to have some "rest" when you reach the top. You should never lose the tension on the muscles when you perform any exercise.
As I said, a very common variation that you can adopt is to use dumbbells to perform the curls.
I won't repeat the execution because it is exactly the same, only with dumbbells.
My advice is to try both versions, try the EZ bar first. If it bothers your joints then use dumbbells. You can use both arms together or alternate.
Preacher Dumbbell Curls
Alternated Preacher Dumbbell Curls
Single arm preacher curls is another good variation to the two versions seen above. It's executed with one arm at the time.
The common error that people do is to help too much the movement with the arm that is not working, by pushing on the pad.
Again, if you need to do that then use lighter dumbbells. You don't make any gains if you cheat.
This is also in every gym and it just guides you through the correct movement.
Machines are usually less effective than free weights because free weight require you to balance more and this factor makes the preacher curls with dumbbells more complete.
But I am not saying that the machine is not good. It's probably better than free weights if you don't execute the exercise in the correct form.
If you use machines you have less chances to cheat!