Noob here. Unclear how to flash different distros

[icantremember]

Hi there. I'm new.

I see what appear to be conflicting reports of what I need to do in order to boot an img from sdcard. 

Seems like it's some combination of:
a) erase emmc
b) hold the power button during boot
c) hold a reset button under the back cover
d) just write an img to an sdcard, pop it in and restart

Can someone please enlighten me? Thanks!