For a serial JB, you can essentially repurpose the
emergency-bridge-recovery.zip package from the JailBreak snapshot.
Unzip it to /mnt/us, and then run
sh /mnt/us/RUNME.sh.
Reboot, then install the full bridge (either the bridge .bin via MRPI; or the hotfix .bin via UYK).
Finally, reboot again

.