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The text below is my own method of a soldering technique on electrical items; it has been gained from a lifetime of soldering both for hobby and industrial work.

Let’s make a start…. For everyday electrical joint soldering a 25 watt iron with a small to medium sized bit is all that’s required. But a larger wattage iron with a suitable bit is just as acceptable and at times an advantage!

To make a good quality soldered joint, heat the iron for at least five minutes. Don’t rush this, the irons tip must be up to full temperature.  Have to hand a damp, soldering iron's tip cleaning sponge pad. If you own a soldering iron stand it is likely it came with a sponge.  If not, then cut a piece of ordinary non synthetic sponge and use that. Remember to keep the sponge damp.
Once the iron is hot, wipe the tip onto the sponge to remove all previous oxidization and any old solder residue. Assuming the tip is in a good condition and it must be! Apply a little rosin cored solder to the tip to wet (coat) the tip in solder.

On electrical joints never use solid stick type solder nor most types of flux - Avoid most paste or liquid fluxes, as these all contain a mild acid which over a period of time causes high resistance problems within the soldered joint. Solid stick type solders and most liquid fluxes are normally reserved of the solid sheet metal soldering jobs – Loco building, rolling stock construction and plumbing etc.
If you must use flux for electrical joints in addition to what is already inbuilt in the rosin cored solder, then ONLY EVER use a special flux designed for electrical soldered joints. It does not contain any harmful acids. Most non electrical specific fluxes need to be washed off under running water. This isn't possible with an electrical joint! For electrical joints if you really need to added flux then select items like Carrs Orange Label, DCC Concepts Sapphire or a liquid flux sold under the 'Re flow' soldering work banner.

For jointing two or more wires together… Dry assemble the joint - It must be cleaned too, use ideally a fibre glass pencil or scrape the surfaces of both components clean, unless it is a freshly stripped wire where the sheathing has kept the surface of the wire nice and clean. You can either pre tin both stripped wire ends or twist them together before soldering and then solder the whole joint.
With the irons tip coated in liquid solder (wetted) place the irons tip directly onto the connection. Wait a few seconds for the heat of the tip to transfer into the wires and then apply a little more rosin corded solder onto the heated joint, if the solder fails to flow into the joint after ensuring the joint is heated fully, apply the solder to both the tip and the joint so as a solder flow into the wires commences. Then as necessary feed more of the solder onto the actual joint, you should see the solder start to melt and flow into and around the joint. Once sufficient solder has flowed into the joint remove iron and do not move the two soldered items for some 5 to 7 seconds allowing them to cool and the solder to solidify.

Soldering wires to the bottom or outside of the rail is the same principle, but here you will find pre tinning both the end of the wire and the pre cleaned place on the rail where the wire is to connect to be the best method.  Pre clean the rail with the aid of a fibre pencil or other means - file etc. Tin with a little corded solder, by coating with solder both on the place on the rail and the wire end.
To 'Tin', apply a small amount of solder to the clean irons tip, then touch the iron onto the area to be tinned. Solder will flow from the tip onto the area. This should only take approx. 1 to 2 seconds. If insufficient solder flows, keep the irons tip in place and feed a little more of the cored solder into the area. With a wire, place the pre wetted irons tip onto the end of the stripped clean wire and feed a little solder onto the tip and wire, it will flow all around it and coat the wire. Remove iron and wipe the tip on the damp sponge. Once every item has been tinned, place the stripped wire end, which if necessary has been pre bent to a small ‘L’ shape, up to the solder on the rail. Apply a little solder to the irons tip and place the iron on top of the wire and lightly press down towards the rail. The hot solder on the irons tip will cause both the wires solder and the rails solder to melt together into one. If necessary, feed a little more cored solder onto the wire with the iron still in place, but normally there should there be enough solder on the wire and rail to make a solid connection. Carefully remove the iron and ensure the wire maintains in contact with the rail and doesn’t move, waiting for 5 to 7 seconds to allow the soldered joint to cool. The use of a small screwdriver blade or even tweezers to hold the wire in place until the solder solidifies and prevent your fingers burning is an option I often use.

The use of metal crocodile clips or any similar sprung metal clamps fixed onto the rails just either side of the soldering work area are advisable, as these act as ‘mini heat shunts’ and help prevent the rail being overheated away from the soldering area which can, if the heat is allowed to be transmitted along the rail, subsequently causing the plastic sleeper fixings to melt.

What you should end up with by either method is a solid, clean joint.   Sometimes the PVC sheath on the wire/s being soldered will shrink back a little. This is a nuisance at times and is due to a) The wires PVC sheathing having a low temperature range or b) Too much heat applied to the joint for too long a period of time. There is little that can be done to stop this other that using higher melting temperature wire insulation or being prepared to apply a suitable insulation further over the wires joint. I like to use Heat shrink tubing whenever possible but at times PVC tape or other insulating material has to be used.

Before you go onto solder another joint or you have completely finished and before you disconnect the iron, clean the tip again on the damp sponge. You will get many years of use from a soldering iron and its tip if you keep the tip clean!

One thing that I have found for the beginner to soldering is to obtain a spare scrap piece of track or a piece of wire and practice, practice and practice until you feel comfortable and at ease with soldering.

A Final Note: Lead free solder requires a higher temperature for the soldering to be successful. Some older soldering irons or those with a low wattage element will not be able to make a satisfactory soldered joint when using lead free solder. Ideally, and I always recommend the use of a lead content solder e.g. 60/40 type with the rosin cord flux inside it. Lead content solder is still sold and it is not illegal to use, except where a soldered item is sold on commercially then Lead Free has to be used under EU legislation.. For lead free soldering use a newer and ideally temperature controlled soldering iron which will have normally a higher working temperature. But do remember whenever using lead content solders to thoroughly wash your hands after its use.