How to?

Down in "Slide and Alternate Tunings," there's a thread titled "another new joiner" where a guy just came in with a question about replacing a broken tube in an old Dobro amp. It and my reply would be better split out into a new topic in Amplification, but when I tried the topic splitting thing I had to either pick a topic to merge it into or give a topic number ID for the posts to go into. Didn't know what to do, so I left it alone and got out.