The difference between this photo and the previous one is that now both of the wires are soldered. You won't need to add solder -- the fact that all the wires are tinned means that there is already enough solder on them. Place two in contact with each other and just touch the soldering iron to the pair. You might need just a tiny dot of solder on the tip of the iron first, to help heat transfer to the wires. But then, very quickly, the solder on both wires will melt and fuse together. Remove the iron and hold the joint still for a couple seconds until the solder cools. That solder joint is done!