I found a new job and need to quit my current one but I don't know how to tell my manager.

Hey all!

Ok, so I started my job the last week of August and it's not bad but it definitely doesn't pay enough. I asked for the manager position and I was denied because a worker who initially didn't want it, asked for it so they gave it to her based on seniority. That's fine. However, my old boss and I talked and he was estatic to know I lived closer and had more help and we agreed I come back. I make a out $300 a week here but I'll make $800 a week there, so it's a not brainer at this point. However, in scared to put my two weeks in but both jobs are requiring weekend work and I can't simply do both. So, I have to leave this current position. The manager who hired me, is here today but she leaves for surgery tomorrow and I don't want to stress her out. My other manager who is also my college friend suggest I want until she gets back from surgery before I do leave. I'm also not mentioning I am leaving because of another job. I am in school and will be starting an internship soon so I was going to say this was the reason. I just feel guilty, I JUST started. I don't know how to word it, I don't know what to say. She's super easy going but now my manager (friend) is also leaving so they're currently down to people but I can't stay because it really isn't my problem. I want to leave on a good note, though. The manager going out for surgery will be out a week and I'm scheduled to start the next job the first weekend of November. I have time to put my two weeks in but not a lot.

Advice on what to say!

TLDR: I'm looking to leave my current job because they only pay $300 a week and I was offered a different job that pays $800 a week. I don't know what to say or do to leave this current job since I only started in August.