anyone installed Android custom ROM on mobile?

Has anyone out there flashed a custom ROM on your Android mobile phone? A friend of a friend has asked me to do this for his Samsung Galaxy S7 phone, so he can use it on any mobile carrier he wants.

I've never flashed a custom ROM before. I've been meaning to do it on my own Android (I hate the bloatware phones come with). The only reason I haven't flashed a custom ROM on my current (or past) phone is (1) fear of bricking my only phone (boo...), and (2) sheer laziness and procrastination.

Although I'm not completely up to date tech-wise, I'm not clueless... I've installed Linux on a few machines over the last decade; I'm ex-IT -- in my previous career/life, I used to do tech support and build/repair/upgrade PCs... I've replaced keyboards and screens on laptops and netbooks in the last several years... and I always upgrade the RAM in new laptops/netbooks because they never come with enough/maximum RAM (and I refuse to pay someone to be my nerd when I can be my own nerd).

So if you have... please comment on your experience. I'm curious as to how tricky/easy it was for you.

TYK!