Oh, no. Now that you have a trackball-optional custom recovery, you are good-to-go.
Just follow these basic steps when flashing a new ROM:
1. Make sure you have a good (100% preferred) charge on your phone
2. Boot into your shiny-new custom recovery
3. Make a Nandroid backup of your current setup (Backup/Restore, Nand backup, ...)
4. Before flashing a new ROM, do your wipes as required (Wipe, Wipe Data/factory reset; you might also want to do a Wipe Dalvik-cache just in case you are switching between Froyo and non-Froyo ROMs--doesn't hurt to always do this)
5. Return to main menu (couple of volume-down presses) and select "Flash from sdcard" and flash your ROM
6. Return to main menu and "Reboot system now" and enjoy your new ROM
This post
http://androidforums.com/eris-all-things-root/127861-universal-eris-root-dummies.html#post1186824 in erisuser1's thread is an excellent guide with images to further elaborate.
Be sure to read all of the developers notes in the ROM thread for any special installation instructions or gotchas.
Good luck!