How to install a Custom ROM

To update my android phone i found out that it requires the installation of a custom ROM. Which is something way out of my comfort zone. I’ve never done it and I’m afraid that by doing so it would be a risk to my phone’s data.
Any suggestions…?

What sort of handset are you looking to flash I.e what is the make and model? There is a process for most - some can be more tricky than others.