Go to https://twrp.me/Devices and download the correct version TWRP for your device.

Boot into boot loader menu

Plug you phone into a computer that has fastboot installed and run

sudo fastboot boot Downloads/twrp-3.3.0-0-sailfish.img

Note you may need to unlock your boot loader.

Your phone should now boot up into TWRP

Swipe to Allow Modifications

Select advanced

Then hit ADB Sideload.

Swipe to start the sideload and then sideload from your computer

adb sideload image.zip

More detailed explaination
https://www.cyanogenmods.org/adb-sideload-twrp/