On your phone, the volume buttons and power work together to act like an old-school feature phone mouse.
Using the volume keys to move up or down, select HBOOT.
Quick tap on power button will make it so.
In hboot menu, select Recovery.
Quick tap on power button will make it so.
The TWRP screen appears, then the choices.
On your PC, move or copy the rom zip file that you've downloaded to the sdk-tools folder using your Windows Explorer.
Do not unzip or WinRar it - just move it intact to sdk-tools.
You can right-click it to get properties and then use your mouse to copy its name out of properties precisely (depending on the Windows version, sorry), and this can save you typing in a moment.
On the phone, TWRP menu, choose Advanced.
Choose adb sideload - slide to begin process.
In the command window, either type exactly, filling in the right name -
adb sideload name-of-rom-file.zip
Or -
adb sideload(spacebar)(use Menu/Edit/Paste trick to put in name)(hit enter)
Report back what happens.