If I generate a seed on my own, how can I use it on ELECTRUM wallet? AS I know it generates it's own seed whenever we create a new wallet and does not allow us to type our own seed.
don't do it. you can not generate a truly random seed on your own, specially if you do it with hand or do it with a broken code that you make yourself.
also you can no longer use an arbitrary seed in electrum, you can read the sticky (
https://bitcointalk.org/index.php?topic=153990.0) about it.