The lead needs to be clean and bright to enable the solder to stick to it. If it's fairly new lead it may be solderable without further preparation. However, if the lead is dull and oxidized, you should scrape the lead in the area to be soldered with the blade of a lead knife.
Then apply the flux.
The iron is held over-handed (as you would a bread knife) in order to get the handle low enough to have the tip flat on the lead. You can allow the weight of the iron to press gently against the joint to transfer the heat into the lead or foil.
The solder is fed to the iron tip so as to melt an approximately five millimetre long piece of blowpipe solder. Move the solder away as soon as it is melted, so it doesn't become attached. As soon as you see the solder at the joint melt and spread, slowly lift the iron straight up.
|This image represents the principle of soldering any metal, not just computer boards|
Avoid "painting" or dragging the iron across the joint. The object is to have a shiny, smooth, slightly rounded solder joint. There should be no points sticking up from the solder joint. If a solder joint is not satisfactory you can re-flux and re-solder. Don't apply too much solder. It's easier to add more solder than to remove excess.
The most common concern is whether there is enough solder on the joint. Very little solder is required to stick the joints together. Often a securely soldered joint shows the ends of the cames. For cosmetic reasons it is usual to use enough solder to disguise the ends of the cames. It is not a structural requirement.