Get some flux aka soldering paste, it helps a bunch getting solder to stick to things like 510 connectors, and makes all the soldering much faster so you don't have to hold heat on it.
I dip the tips of my wire in the flux, and melt a little solder onto the top of the iron, and tin the wire by applying the melted solder to the bare fluxed wire, it evenly coated the wire in a very thin layer of solder.
Then I dip in flux again and melt some more solder onto iron, put my wires together and apply iron, it only takes a couple seconds.
Makes a very strong connection.
Same basic principle for soldering other connections including circuit boards, except with boards, you don't have to tin the holes, but a tiny bit of flux helps make it stick without needing to hold the iron on it.
I don't use resin core solder, just some solid lead free solder for electrical use from the local hardware store, works great for me.
Be sure to keep the tip of your iron clean, once it's starts to gunk up you will notice it doesn't transfer heat very well and makes everything harder.
Once mine gets a little crust built up, I sand it down to the metal with a little emory cloth or the metal file in my leatherman.
I then like to stick the tip of the iron in my flux paste and melt a little solder on the exposed copper to keep it from oxidizing before my next use.