Don't practice arguing. Practice thinking.

I would not have argued in a rhetorical way, but would have tried to present the most thoughtful arguments that I could identify. (In other words, it would be a way of thinking about the issue.)

The exercise is to relate to the position and then experience (from a relatively detached viewpoint) how the position is taxed or not taxed by the counterarguments. I think this would be an interesting exercise to see if your arguments would work with someone who (in theory) holds your friend's position but is willing to absorb the counter-arguments.

Thanks for your advice, but it doesn't resonate so much for me. I'm not fearful of persuading people of things, but I need to learn how to persuade myself. In particular, what you wrote seems backwards. Believing the right things for the right reasons follows from developing and recognizing good arguments, and I need more skill in developing arguments.