All you have to do is root your Evo and then flash the ROM that you want, but to tell the truth I'm not exactly for sure if you can flash from 2.3 back to 2.2, but it won't hurt to try and it won't do anything, worse case is that it will tell you that you can't downgrade as the software your running is newer.
I'll put up some links so you can read about rooting, and a link to a thread with stock ROM's. Both rooting and flashing a ROM is pretty simple, but I understand that it makes you nervous doing it, I was the same way when I began.
When you root your phone with Revolutionary at the end of the process it will ask you if you want to install the recovery, pick yes as that's important in case you want to get to the root of your phone. The ROM you just put on your SD Card and it will auto flash it if it's a PC36IMG file, or you can run a RUU from your laptop/desktop, or a zip file from the recovery menu. Below are those links I was talking about.
Link on how to root your phone:
[ROOT] EVO 4G Gingerbread 2.3 Root Tutorial - xda-developers
http://forum.ppcgeeks.com/htc-evo-4g...roms-wipe.html
Link to ROM's (this is the newest of the old 2.2 ROM's):
http://shipped-roms.com/download.php...&send_file=yes
***Note that the above link is to a ROM file that is a RUU and will be installed from your laptop/desktop
If you are interested in a 2.3.3 ROM that is just about flawless, runs extremely smooth and leaves you lots of space for your programs, excellent battery life, and has a lot of Ice Cream Sandwhich (ICS) touches I can give you a link, will still need to root your phone if you're not rooted already, pm me or leave a message in here if you have any questions, good luck.