You can go directly from Froyo CM using the Team Black Hat GB update zip (available in the paid version of the TBH app, I believe), but the "normal" process is:
1. SBF from Froyo to rooted GB leak
2. Install CM4D2GB build from Rev
Detailed steps that were posted by Simon:
heres the process i took to install CM7GB
1. flash tbh stock froyo .zip file (wanted a clean start to be safe)
2. boot into CWR
3. wipe dalvik/cache/data (just to be safe...i know i paranoid)
4. flash GB leak file and reboot [Redflea - he's referring to the GB zip file that acts like an SBF that Team Black Hat provides via their Team Black Hat app. If you don't have the app (it's paid) you can flash the rooted GB leak from here:
Droid 2 Gingerbread (2.3.3) - Release - PreRooted - the "Direct Download" link in the second set of 1-4 steps in P3Droid's post]
5. boot into CWR
6. repeat step 3
7. flash CM7GB and reboot [Redflea - get the most recent build from Rev's dev thread here:
[DEV_THREAD]Droid 2 CM7 with Gingerbread Leak Kernel]
8. boot into CWR
9. flash gapps and reboot (im sure i couldve flashed after CM7GB but i wanted to make sure it booted up first)
Once you flash to GB you'll have to SBF back to Froyo if you want to go back to CM74D2 Froyo...it's easy to do, but takes a little time.