Hello Muhammad,
I suppose from other your thread you already have between the C3750 stack and the C3550.
You can start by configuring the two additional links first in shut on both ends.
You can then do no shut both sides using the working still stand alone link.
After STP convergence you verify the bundle with
sh etherchannel summary
sh cdp n
you can shut the standalone link both ends add the command channel-group1 mode on then renable it on both sides.
I would use LACP instead of on mode as a protection mechanism
to use LACP you don't configure the port-channel interface and you use
port-channel mode active.
However, you need to check if the C3550 supports LACP and also if it supports it the commands can be different
You need a maintanance time window for safety.
Hope to help
Giuseppe