All you need for this exercise are dumbells and a set bench. The great thing about preacher curls is that your take away any possible swinging you may do during regular curls, and these isolate your biceps more.
1) Sit at a preacher bench.
2) Keep one arm rested on the bench, fully extended. Rest only the upper arm.
3) Curl the dumbbell upward toward your face. Then slowly lower the dumbbell back down, but not fully extended. Repeat.
If you have any additional tips for this exercise, please let me know in the comments below and I will add them to this card and tag you in it. :)