Note -1. Before Doing These Steps Your device bootloader should be unlocked. otherwise you may brick your device.
2. Install Adb drivers on windows if you are using windows or if you are using linux install adb drivers in linux
1. Download Custom Roms i will provide the links by the end
2. download TWRP
3. Example iam On Stock rom
4. Now i want to Flash Any one of custom rom but how?
> Be ready with Any custom Roms from below example i wilL Flash Pixel Experience Rom
> Be ready with twrp-3.3.0-0
>Turn of your phone and switch to download Mode & Connect phone to pc
4. Now flash twrp using cmd command "fastboot flash boot twrp.img"
5. now your phone boots to twrp
6. now copy the custom rom to phone using twrp dont worry mtp working
7. flash the rom and wait or 5 min for installing
8. now check your slot example iam on Slot A
9. now tap on reboot to recovery now you will again boot in to twrp
10. now change the slot from A to B by tapping reboot ,slot B
11. now you are on slot B now flash gapps for all the roms except Pixel Experience and flash magisk for root if you want using magisk manager zip file
12. now click on wipe & advance wipe now wipe all except system & vendor and tap on reboot system
13. now you people will see pixel experience Boot animation wait for 7 min to boot up and the rom is yours and same method for all custom roms installation