Resigning seems hard, but it's a little bit like giving regular feedback in my opinion. First of all it's best done face to face, because you don't want to be misunderstood, and you want to give the other party a fair chance at replying.

Second, you give the compliment sandwich. You start out with thanking them for the opportunity (any job is an opportunity, no matter how shitty it was, you made money and you learned something, sadder but wiser you may be). Then you tell them you would wish to quit your job after the contractual resignation period. Don't neglect mentioning that! If you just say you want to resign, they might just accept your immediate resignation and you'll miss out on the period you might have planned on. The final compliment has to make sure you'll be able to work together through the final period, just assure them you'll keep up your efforts and help transfer your knowledge to colleagues.

I'll do it for you for $450 if your employers speak English or Dutch ;)