It can be confusing knowing what things to do first and which order to do them in. Here's what I did:
1. S-OFF using xtc clip (its a thing you buy, but its cheap). That will allow your bootloader to install unsigned PG88img updates.
2. Use PG88IMG.zip to flash an engineering hboot.
3. Use eng hboot to flash a custom recovery (I used 4ext recovery)
4. Use the new recovery to flash your new rom. There are many 4.x androids to choose from (look for the first pinned thread in the development forum - huge list of saga-related links) but I'm using the unofficial cyanogenmod 10.2 stable and i'm very happy with it.