Installing a TeamWin - TWRP custom recovery
Steps 1. - 7. as described in the. /e/ documentation
The decisive step starts here:
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Holding the VOL(-) and HOME and POWER buttons simultaneously until the screen goes dark;
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as soon as the screen goes dark, immediately release the VOL (-) button and press VOL(+) and HOME and POWER simultaneously until the TWRP Recovery start screen is displayed;
Note: Be sure to reboot into TWRP recovery immediately after having installed the TWRP custom recovery. Otherwise the custom recovery will be overwritten and the device will reboot (appearing as though your custom recovery failed to install) - usually the stockrom Android recovery mode is then displayed again.
The /e/ documentation would be clearer with a few some additional terms (added by me in bold font).