Gmail forwarding and thread merging

Question

In this question about merging threads in Gmail I was given this advice:

forward an orphaned email to yourself as a response to the thread you want it to be a part of, then delete the original

How do I do this?

I know how to forward an email but how to do it so that gmail understands that it should belong to another conversation?

Answer 1

In order for the message to be threaded with another conversation, two conditions must be met. You must make the subject similar (ideally using the same subject is best), and you must use an email account that exists in that thread (for example if the messages are sent to your account, foo@gmail.com, you should send the message from foo@gmail.com.)

See: How does Gmail decide to thread email messages?