edit: 0) Obviously backup everything you can including SDcard. Put files on SDcard ready. Bear in mind you might get confused and a nandroid restore might not be working so keep files in an accessible form in adddition to a nandroid if you can. nAndroid = backing up via the CWM recovery system.
1) Flash 2.60.20 sbf via RSD lite.
2) You now have a bootloop. Battery pull, hold rm and plug batt & cable back in
3) at the ! sign press @ key. You have the locked 3e bootloader but this allows to wipe flash and cache. Wipe all that you can
4) Now boot into verizons bloated firmware and root with Droid 3 easy root
5) Once rooted finally load a decent CWM on there via Droid 2 recovery bootstrap apk using adb install d2whatever.apk then you need to go to apps and actually run it. Then hit the reboot button in that app.
6) Now can reboot into CWM with R+M + plug usb (more reliable than pwr button I think) and flash Apex. ICS isn't perfect at the time of writing (well, GSM can work but it's confusing so let's keep things simple...)
7) I think at the time of doing the ROM you also want to do the band unlock and GApps?
List of useful files in order:
DROID 3 easy root script v7-xp
Droid 2 Recovery Bootstrap v184.108.40.206.apk (appears to be $2 on the market)
Around step 7 I'm not yet completely clear on myself yet. Which first? ROM, camera fix, band unlock then kernel?
1) Both volume keys for recovery in GB whereas in Froyo it was R + M + Power. ICS?
2) Power + X = bootloader?