all you need
1. unlock boatloader(depend on unit if supported some device are not supported like huawei and etc.)
2. custom recovery
---- check if may available custom recovery for your unit if wala hanap ka lng sa site if wala talaga not possible ang custom recovery and custom rom
--- flash recovery thru adb fastboot and vbmeta files
3. custom rom
------ wipe 5
---- flash rom thru custom recovery(follow instruction needed for your prefer custom rom para iwas brick ng device
---clear data.
basically ganyan ang mag custom rom simple lng pero mararanasan mo mag deadbloat phone once pero worth if mag succes ka..