You can use the "Install Rom from Sd Card" option in ROM Manager ( do NOT check any options when doing prompted) , OR reboot into Bootstrap Recovery and do it manually.
They both do the same thing, ROM Manager just boots into Bootstrap and installs the file automatically.
This is the same for any properly formatted "update.zip" style Theme, ROM, update, Patch, Hotfix, etc, etc.
HOWEVER, I highly recommend doing ti manually, at least until you become more knowledgable. By Booting into Bootstrap Recovery you will not only learn how to use the recovery, BUT you can also watch for errors (and inform the Dev if there are any) in the update.zip files. Using Rom manager it happens so fast you will never see them and a failed zip install can cause a lot of confusion or even worse.
For example, if you run some Roms out there that format your system area and then the rest of the install fails...using Rom Manager it will typically reboot automatically and guess what...you have no system area...Bad Mojo!! If you did the same thing manually, you could see an error, then you can restore your last good backup and reboot manually and still have a working phone
FYI, NONE of my Themes or any update.zip I make will format your system area.
Also, anytime you have a problem and need to use recovery you will already know your way around it and not be in a "double" panic from the phone problem compounded by having no clue of how to use recovery to fix it.
FYI, my standard for using Rom Manager instead of manually using Recovery ...is when you can navigate to where you want to be in Recovery..with you eyes closed(seriously)...then and only then start the habit of using Rom Manager..AND even then, only on update zips you know are safe. I would never use it on a new update zip I haven't used before.
