Just some rough notes. I wanted an official way versus using hacks and exploits to avoid viruses and what not.
I skip rooting your existing ROM since I assume you will be overwritting it with a custom ROM anyway. Rooting would just involve installing the SU and Superuser.apk anyway.
- HTC Phone with SD Card and 300MB free space
- Okay with wiping all data and settings from the device!
- Android SDK
- You just need the fastboot command and not the platform-tools or anything else really
- HTC Sync if you are using Windows since you'll need the drivers
- A Recovery Image
- For HTC Evo 4G just pick the one for "HTC Evo"
- Look in the second column for the download link
- A custom ROM
- An HTCDev.com account - free and home of the official HTC developers
- Download your custom rom zip and store it on your phone's sdcard in the root of the sdcard
- Download and include the gapps zip if you want it
- Power off your phone
- Remove the battery (or disable fastboot in the OEM firmware: home screen, menu, settings, Power, Fastboot or settings, Applications, Fastboot)
- Hold down the Volume Down button and Power button until the phone turns on
- Ensure your USB cable is connected
- Follow the instructions at HTCDev
- For the HTC Evo 4G use "All Other Supported Models"
- Once unlocked don't reboot, continue to next section
Booting custom recovery and flashing the custom rom
- fastboot boot customrecovery.img
- Where customrecovery.img is the name of the local image on your computer
- Use the custom recovery to flash the zip custom rom copy on your sdcard to your device
- Make sure to wipe all data and cache with the custom recovery you booted otherwise you will probably get an endless boot error
- You don't have to flash the custom recovery image if you don't want too. I did so after I booted my custom ROM as that was easier.
- Reboot and enjoy
- If you get errors with fastboot not finding your device try running it with administrator privelege