I'm 27 and want to start really investing monthly. Help me start?

Hi! I'm 27 and had a rough go up until this year with consistent income. I finally have a more consistent income and really want to start investing $300-$400 a month. I keep seeing things about a 3 fund approach - is this something I should do? I already have a Roth IRA through my job and have 15% taken out of every pay check to automatically go there. Is there anything else I should do with that?

Should I invest in mutual finds, Index funds, ETFs etc?

Do you prefer Fidelity or Schwab?

I feel like there's so much confusing info out there and I've tried to research but I'm just so lost on where to truly start and how to keep things simple. I would love for the money to grow faster than not, but in the end of the day I know I have to be in it for the long haul. I know it's already late for me to start, but at least I'm able to go for it now.

Give me your best simple steps and talk to me like I'm in third grade so I can just do it, get it set up, and leave it alone. Thanks!