The individual mods should generally contain information on how to install them. But the process is pretty much the same for both full UIs and individual mods: you'll put files in a folder and reference it in your eq2.ini. Any files not found in that folder the game will automatically pull from the default UI.
If you have specific UIs you're talking about that don't have instructions, give a link to them and we may be able to give you more specific help. One thing to keep in mind is when UI mods were last updated: if it's too long ago they may not work (and can result in EQ2 crashing and other bad things).
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