When you say install a Mod, I assume you mean a custom ROM. If that is the case, it is good news for you. You actually WANT to have Clockwork Mod (CWM) on there. You are in the same boat I was. Here is what I did:
1. Spend some time reading all the great info about rooting and ROMing the Epic in these forums and around the web. Go to YouTube and look at qbking77's videos.
2. Install Titanium Backup Root and/or MyBackup Root and back up apps and data.
2.1 If your launcher has an option to backup, do that. Consider taking screenshots of your home screens to help restore later.
3. Make sure your battery is fully charged before doing a lot of this stuff.
3.1 Download the Gingerbread ROM you want onto your SD card root directory (or do it later via your computer)
4. Get familiar with the three finger boot method. Turn off phone, turn back on pressing down volume, camera and then power. Hold all three down until capacitive button lights come on. This will take you into CWM.
5. In CWM, you can make a backup, also called a Nandroid backup. This is a full backup.
6. Back up entire SD card contents to your computer hard drive (just to be extra safe).
7. I had an older version of CWM on there, so first thing I did was flash a newer version per a qbking77 video (I have 3.1.0.1.) There is a newer version now (5 something?), but I'm not sure it is as stable as older stuff---again, read, decide for yourself.
8. Here's where it gets a little stressful: In CWM, wipe the cache partition 3 times. Under Advanced, wipe the Dalvik cashe 3 times, and then wipe data/factory reset 3 times. THIS WILL DELETE ALL YOUR DATA!
9. Flash (i.e. install zip file in CWM) that ROM you put in your SD card root directory
10. Start rebuilding your phone. Be sure to have usernames and passwords for Google and everything else. Best to download apps from the market and restore data from Titanium or MyBackup I've read, especially when going from one OS version (Froyo) to another (GB).
11. Be happy with how much better your phone runs.
You can also think about changing the file system from RFS to EXT4 at this time, but I'll leave that research to you. The other thing to think about is SD card cleanup. Once up and running, I made sure all apps were loaded on the phone, formatted the SD card, then copied media and other stuff (like Titanium backups) from the computer backup.