Well you won't get that kind of guide, cuz only small part of users still using p990 (my guess) got back to stock, so all the guides are a bit outdated but shouldn't brick your device. Saying it short, use the old guides
Use smartflash to install an old baseband like 20S. Then use LG update tool to flash the latest stock for your region. Alternate, use AIO tool to flash both baseband and stock. You will need to download .kdz file and extract it to fls and bin file in AIO tool prior to flashing. Fls are baseband and bin are the rom
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