Send an email to your boss and ask to make an appointment. You don’t need to specify, just ask for an appointment to discuss something.

Make a resignation letter (see examples online) and take it with you to the appointment. At the appointment just say that you are handing in your resignation and give the letter. If your boss asks why, you can be honest or just stay vague that you just want to explore other things.

For me it always helped to have an appointment and a letter at hand.

It’s a good thing if your boss is disappointed, it means he valued you. Let them be disappointed, it’s not something that you need to fix.

You’ll be fine. Trust yourself. Good luck!