The initial installation (flashing) of Sailfish OS is done according to the instructions in the Jolla Shop. This is based on using the flashing script in a similar way to the earlier Xperia phones. The script is included in the downloadable zip package.
There is, however, a problem (in OS version 5.0.0.61) with the subsequent flashing of Xperia 10 IV and Xperia 10 V phones: flashing them the 2nd (3rd,…) time does not work with the flashing script. Instead, the following set of manual commands shall be used - invoke them in your Sailfish directory:
$ fastboot devices
$ fastboot flash boot_a hybris-boot.img
$ fastboot flash boot_b hybris-boot.img
$ fastboot flash dtbo_a dtbo.img
$ fastboot flash dtbo_b dtbo.img
$ fastboot flash userdata sailfish.img001
# Take only one of the next two commands, depending on your phone!
$ fastboot flash oem_a *_v3a_murray.img ## for Xperia 10 IV (XQ-CC54)
$ fastboot flash oem_a *_v3a_zambezi.img ## for Xperia 10 V (XQ-DC54)
$ fastboot reboot
Alternatively, revert the phone to Android with the Emma flashing tool and install Sailfish OS with the flashing script in the normal way.