How does goomanager work? I want to install custom ROM on my HTC One XL. I have unlocked (not rooted) the phone following the unlock process on www.htcdev.com. Now I installed goomanager which supposed to let me install TWRP recovery.  

I already downloaded openrecovery-twrp-2.3.0.0-evita.img and copied the file to my mobile but it doesn't seem to me that I can use this downloaded file. 

More over it seems to me that I can directly use goomanager to install custom ROM and not to do it via TWRP as I read somewhere.

Q1) Can I install files that I downloaded using my computer and copied over to my mobile or goomanager will always download them for me?

Q2) Do I have to install TWRP to install custom ROM (cm10)?

Q3) Can I use goomanager to install custom ROM?
Though I wouldn't usually recommend device specific websites but since i'm not a One XL user I cannot verify its authenticity.
Next time when you ask questions about custom roms please mention whether you are already rooted, locked or unlocked bootloader(htcdev.com only unlocks your bootloader without giving you s-off). Try this to just get an idea of one of the methods to install a rom.

http://onexroot.com/one-x-roms/how-to-install-custom-rom-on-rooted-att-htc-one-xxl/

goomanager is a service used by developers to upload their roms into one location and users can check for available newer versions of existing roms. Follow the regular clockworkmod / twrp procedures to flash your rom.

You can always copy roms that you downloaded with your computer onto your phone.