depend on the device type, for some (like my old samsung s4) official instruction is to install the app from the store, then patch the boot.img include in the /e/OS.zip using the app, and flash it to the device.
without TWRP, it is possible using fastboot or for Samsung devices you can use heimdall, heimdall-front-end,thor, Odin3 windows, odin4linux or Galaxy-Flasher, the ultimate graphical tool for Samsung devices
