Some tips for soldering beginners that helped me when I first started building FPV drones.
-Get good solder. Kester 63/37 is my favorite. Stay away from lead-free solder, it's harder to work with. Soldering temperatures are too low to vaporize lead, just wash your hands after handling leaded solder.
-Buy good flux. If you have a problem soldering, flux will probably fix it. No, the flux inside the solder wire is not enough.
-Apply flux on the pad, heat the pad, lay your wire on the pad. While applying heat to the pad with your soldering iron, start feeding the solder onto the point where your pad and wire meets. It should start melting immediately and create a shiny connection. If it's not melting immediately, your soldering iron might be too cold. I like 380-420 range. Higher the iron temperature, quicker you should be. Don't rest the soldering iron for a prolonged time on the pad, it will burn it. Nothing on a flight controller (including XT-60 power connections) require more than a couple of seconds to heat up.
On a side note, if you're not going for a specific build, don't buy rebranded brushless motors with pilot names on them. The same motor usually can be found cheaper, with the factory branding. Same goes for solder. TBS doesn't manufacture solder, they just probably rebrand Kester or some other similar brands with a margin. Joshua Bardwell's FPV shopping list should mention every one of those brands so you can look it up.
Also as an additional step, consider conformal coating after you're done with all your connections.
As long as you wash your hands after soldering, leaded solder is fine. Lead-free solder is harder to work with and never comes out as good as leaded solder.
On a side note, if you're not going for a specific build, don't buy rebranded brushless motors with pilot names on them. The same motor usually can be found cheaper, with the factory branding. Same goes for solder. TBS doesn't manufacture solder, they just probably rebrand Kester or some other similar brands with a margin. Joshua Bardwell's FPV shopping list should mention every one of those brands so you can look it up.
Also as an additional step, consider conformal coating after you're done with all your connections.