Select the correct size soldering iron for the application

If you are soldering small wires (about 20 gauge) then you should use about a 20 watt soldering iron. If you are soldering battery packs or larger (12-18 gauge) wires then you need at least a 40 watt soldering iron with a thick tip to have sufficient heat to solder properly. The Weller SP40L costs less than 20 dollars and is a very good soldering iron for soldering thicker wires. If your tip is too narrow or small, then the tip will not transfer heat to the wire or connector very well, and when you touch the solder to the part, it will not melt or flow well because the temperature is too low.

Use the correct solder

You need ROSIN core solder for soldering electrical connections. Do not use acid core solder, because this solder is for applications such as plumbing and the acid will cause corrode the wires and connectors.

Use the correct size heat shrink tubing (if necessary)

Heat shrink tubing generally shrinks by 50%, so you will need tubing which is no larger than twice the size of the wire and/or connector in order to ensure a snug fit.

Allow the soldering iron/gun to warm up

It requires approximately 750 degrees Fahrenheit to melt solder. If you try to solder before the iron/gun is fully heated, then the solder will not melt properly. If the soldering gun does not heat up properly and/or takes too long to heat, then the screws holding the tip have probably loosened, so not enough current is flowing to the tip. Disconnect the soldering iron, wait for it to cool, then tighten the tip.

Slide heatshrink tubing onto the wire (if required)

In some circumstances you cannot slide the heatshink onto the wire(s) after soldering, so you should slide the heatshink tubing onto the wire first.

Make sure that you slide the heatshink tubing far enough away from the soldering area so it will not shrink prematurely from the soldering heat.

Clean the surfaces to be soldered if necessary

If you are soldering batteries together you will need to clean the surfaces with some sandpaper before soldering. This is not required for gold- or zinc-plated connectors or circuit boards which are already coated with solder, or for wires which have recently been stripped.

Twist the wire conductors tightly together

If they conductors of the wire are loosely twisted then they will not heat properly and it will be very difficult to solder them. If you apply a little extra twist just before soldering, they will be much easier to solder.

Coat ("tin") the mating surfaces with solder (do not solder them together yet). If soldering wire to a gold-plated bullet connector or an Astroflight Zero-Loss connector, then do not tin the connector.

This involves three steps:

A common mistake is to apply the solder directly to the soldering iron/gun tip. This is not the correct way to solder and will create a hole in the soldering iron/gun tip after a while. The solder should be touched to the surface to be tinned. If it does not melt, then the surface is not hot enough.

Touch the parts together then heat with a soldering iron

This step is much easier if you clamp one of the parts with a small table vise to keep it from moving, and hold the other part with a wooden clothespin to avoid burning your fingers.

Alternatively, you can drill a hole in a block of wood to hold bullet or Zero Loss connectors while soldering.

When the the soldering iron is touched to the solder it should melt and merge with the solder on the other surface. After the solder has completely melted, remove the soldering iron tip and let the solder cool. Be sure not to move the parts while the solder is cooling.

Visually inspect the soldering joint

The solder joint should be smooth and shiny after the solder cools. If the solder joint looks dull and grainy, then the solder joint is bad and it should be reheated to let the solder flow properly. You may need to apply a little fresh solder so the fresh rosin will allow the solder to flow freely.

Shrink the heatshrink tubing

Heatshink tubing is best shrunk with a hairdryer on a high heat setting. Allow the hairdryer to warm up, then hold the heatshrink tubing in the hot air stream., Be sure to rotate the tubing so it heats evenly and shrinks without creasing.

Be sure to WASH YOUR HANDS after soldering

Most commercial solder is 60% tin and 40% lead. Lead is harmful to your body and causes lead poisoning which prevents calcium, iron, zinc and other minerals from being properly used by your body. Be sure to wash your hands after soldering before eating!!!

Lead-free solder is becoming much more widely available. If you have the choice you should use this type.