Root How Do I Install a Custom ROM? (MetroPCS/Marvelc)


Hello, just what the title states, I want to know how to install a custom ROM. I am a newbie here and have just recently rooted my device.
I am currently trying to install a custom ROM but I don't know how to "flash" it or whatever. Please help me! Thank you!

-Android 2.3.4
-Sense 2.1