Best advice, get a quality variable temperature soldering iron. 2nd, quality solder, 3rd cleanliness. Both on the wire and the soldering iron. Solder flows towards the heat, on bigger wire, heat on the opposite side that you bring the solder in. If you melt the solder only on the iron, the wire could still be cold. Biggest mistakes, too large of diameter solder that should be used for plumbing with the wrong (or even no) flux and a cheap Walmart tool.