Although I'm pretty good at plumbing soldering, I could not consistently get clean solder joints for bullet and battery connections.
I found this video very helpful, and my connections are coming out perfect
https://www.youtube.com/watch?v=cVPLA9nk9_I
Another quick tip (works for me anyway).
If you're soldering XT-60 connectors, I suggest putting the connector on it's side rather than vertical
Great video. Thank you for sharing.
Guy