Look at the image above.
Let's say the address you're looking for is 0x377A6DA0. (The 2nd address in the picture)
You see the letters/words next to it? Those are your off-bytes.
so for 0x377A6DA0, the off-bytes are: F88F7D00. (<- that's 4 bytes long)
just separate them "properly", like so:
That's your off bytes! Now usually to find the on-bytes, you just NOP it. (Not always that way though)
NOP = No Operation, which is \x90.
Since there are 4 bytes, you would just NOP the 4 bytes to get your on-bytes:
so for 0x377A6DA0
(The address isn't useful, I just got the pic from a different thread and cropped a few addresses out.)
Hope you learned something!