How to attach a liebig condenser to a flask for reflux heating?
Maybe I don't understand your question well, but often in the lab when we do reflux heating, we don't really attach the liebig condenser.

You fix the flask using a clamp. And gravity holds the liebig condenser into place. You can put a clamp around it to hold it straight but you'd better not tighten it too much because these things are fragile.

I hope it helps you :)

If it's not clear maybe it would be easier to answer if you posted pictures of your glassware!