1. Backup all your data to an external source. All your data will be wiped.
2. Make sure to remove pin/password and logout of google accounts.
3. You should have same firmware on both slots. If you are coming from OOS/COS you can either use regional/super flasher to flash oos/cos on both slots. If you are not comfortable with this, local install the current firmware version.
4. Download the custom rom.zip.

=Recovery Builds=
1. Get the boot, init_boot, vendor_boot and recovery.img (& super_empty for Ace5 users ) from the sf folder.
2. Connect to PC.
3. Reboot to Fastboot - Press Both Power_button_key + Vol_down_key
4. Flash the following partitions one by one:
-fastboot flash boot boot.img
-fastboot flash vendor_boot vendor_boot.img
-fastboot flash init_boot init_boot.img
-fastboot flash recovery recovery.img
-fastboot wipe-super super_empty.img
5. Reboot to recovery using "fastboot reboot recovery"
6. Select Wipe Data/factory Reset & Confirm
7. Select 'apply Update' From Adb and execute
-adb sideload <drag-&-drop-rom.zip>
8. Select Wipe Data/factory Reset & Confirm
9. After Installation Complete, Reboot to System.

=OTA update/dirty flash=
If the custom ROM supports local install, you can do that from Settings > System Update > local install.
If not,

1. Reboot to recovery
2. Apply update > Apply from ADB
3. Open command prompt & sideload rom using command
-adb sideload <drag-&-drop-rom.zip>
4. Reboot.