How to flash AOSP ROMs:

1) Download the vendor_boot.img and ROM

The vendor_boot.img is recovery

2) Install the ROM's recovery using :

fastboot flash vendor_boot_ab vendor_boot.img

3) Boot into it using the power + volume up button or

fastboot reboot recovery

4) Wipe data/factory reset

5) In aosp recoveries you have to go into Apply update and choose your preferred option.

6) On your PC with the downloaded ROM, type in the terminal:

adb sideload 

ROM.zip (ROM filename or drag and drop the zip file)

7) Optionally you could flash Gapps or other packages which you'll have to reboot back to recovery (with the option given after flashing the ROM) and repeat steps 5 and 6.

8) Reboot and enjoy