Regaw should of installed TWRP (Team Win Recovery Project) automatically. To boot into the recovery, you can hold the vol down and power button, until the phone flashes off. Keep your finger on the volume down and let the power button go when it goes off, then quickly hold power again. You should boot into the boot menu, you should see recovery as an option (volume is to go up and down menu selection, power to select an option)

To flash you do that from the recovery, follow the flash guides on the XDA pages for the specific ROM's. Though most are Factory reset and wiping the caches before flashing. I'm sure someone else here will explain better. Always make sure to do a backup in recovery of whatever your on.

 

You can hook up your phone to your PC, then copy over the ROM zip (whole zip, don't extract anything) onto the internal SD. When you plug in your phone you'll see a few different options, media sync should work for you and the phone will show up. Just go into that and find the download folder or something, or make your own.

Back up everything you want, clear Google Wallet if you set it up (clear the settings) before flashing to avoid locking it up permanently. As I mentioned, follow any instructions on the ROM pages and just use general caution before doing anything. If you have a specific ROM in mind, look on youtube for an install vid.

When you boot into recovery again, you'll see install on the top left and that is where you navigate to find the ZIP on the phone. You'll see Wipe on to the right of that, you'll want to do that whenever flashing a ROM for the first time or a new ROM in the future. Factory reset, cache & dalvik cache. Please wait for more experiences members to offer advice, but otherwise that is the basics. When you do a wipe like that, apps/app data and contacts are lost (contacts easily backed up with gmail) SMS messages too. You won't lose media like photos or music.

 

Alright good luck and enjoy, definitely look into Meanrom and Viper4GROM for the two of the best out there right now. Depending on the ROM, you can reboot to recovery via the restart menu when the phone is on.

Also look into S-Off method if you haven't already, then you can fully install the new bases that sprint releases for the phone (You can only install updated radios with S-ON otherwse)

[S-OFF] LazyPanda S-OFF - Sprint HTC EVO 4G LTE - xda-developers

and a nifty Mod

http://forum.xda-developers.com/showthread.php?t=1730364

 

Should have no problem flashing with S-On (I am still on it) just gives you the full deal.