The instructions from FP5 don’t fully apply for the FP6 at the moment. To flash LOS: reboot to bootloader, flash recovery.img, reboot to recovery, factory reset, sideload .zip. (All other images are included in the .zip) (See also here). If you want GApps, sideload them as well before first time booting into LOS.
My personal advice: When you switch back and forth between the stock FPOS and LOS, I wouldn’t recommend relocking the bootloader. Because of roll back prevention there is a high risk in bricking the device if you are not knowing what you do. Plus you need to build LOS on your own with the modifications mentioned above or use a fork supporting it.