Always click Download with Manager if I instruct you to Download and install a file.
Occasionally you will be instruction to just Download a file in which case you should click Manual Download.
Switch to the Download tab in the right pane of MO2 and double-click to install a mod.
You can rename mods during their installation. I highly recommend adding stuff like texture resolution to the name, or the name of that particular version of the mod, eg:
“Skyrim Project Optimization SE – No Homes (ESM Version)“
“True Wolves of Skyrim 2k“
When installing multiple files from the same mod page, they will by default have the same name. Change that name and don’t merge those files into one mod folder, eg:
“Unofficial High Definition Audio Project – Voices EN (Pt 1)“
“Unofficial High Definition Audio Project – Voices EN (Pt 2)“
When updating a mod, always click Replace and never Merge.