Don't worry tell them directly before leaving, because if they were firing u they don't hesitate.

Most ppl do it right by submitting leavening letter befor 3 months of leave

I've been there. They didn't want me to leave but I had to, so i wrote an email directly to the CEO explaining why I had to leave and it went well (opposite to my expectations). They even told me i can get back whenever I want in the future. If you have to leave do it without hesitation. If they get disappointed they shoud be by their quality of leadership not by you. So don't worry about them being disappeared (They shoud have done a better job to keep you there if they needed you that much). You are hired so that you can get the freedom to work for whom you think is best for you, so don't give your freedom away.

Be hones and tell the truth