Make sure that the soldering iron is of a high enough wattage to heat the area both sufficiently and quickly. With a too-small iron, the heat will dissipate faster than the iron can provide enough heat to make a strong bond. I use either a 40 or 200 watt iron: either will complete the job in a second or so. Remove a couple of ties on either side of the joint, and use wet rags to avoid melting the ties beyond that. I also like to use a track gauge on either side of the joint area. Some track seems to have a slight oily film on it, probably from the drawing process - a touch of alcohol on a brush should remove that, if necessary. Small-diameter resin-core solder, such as used in electronics, works well. Do not use acid-core solder or flux.
I use a chisel-style tip, placing it atop the joiner and against the inside of the rail, then place the tip of the solder at the same point, but on the outside of the rail. The solder should flow almost immediately, and will be drawn into the minute space between the rails and the joiner - any build-up that shows on the side of the rail is superfluous, contributing nothing to either the mechanical or electrical joint. You can use a file to remove it later, although I find that careful use of a cut-off disk in my Dremel is both faster and neater. Watch closely as you work, and remove both the soldering iron and the solder from contact as soon as you've seen the solder flow.
Modify those ties that you removed earlier, using a needle file and/or knife, to accommodate the extra width and thickness of the joiners, then slip them back in place. When you do your ballasting, the ballast and glue mixture will hold them in place.
Wayne