If you are not familiar with the term Custom ROMs, let me clear things for you. Custom ROMs are modified versions of Android operating system based on the source code released by Google and/or device manufactures.
BEFORE YOU BEGIN
HOW TO FLASH HTC ONE M9 CUSTOM ROM
Download rom and copy the zip file to the root of your SDcard.
Power OFF your HTC ONE M9 and wait for 10 seconds.
Reboot your HTC ONE M9 in recovery mode.
Press Power and Volume Down keys. When the white bootloader appears, choose ‘Recovery’.
In TWRP recovery perform a full data wipe / factory reset.
Use TWRP to install the rom .zip file. Just tap Install, choose the .zip file, and the app will do the rest.
Reboot the smartphone when you’re finished.
You have successfully flashed your HTC ONE M9 with custom rom.