Use fine emery cloth, wet & dry sandpaper or the specially made wire brushes to thoroughly polish the outside of the pipes and the inside of the fitting.
Cover the cleaned surfaces with a thin layer of soldering flux and assemble the joint(s)
Heat the joint with a blowlamp, concentrating mostly on the pipe (as that has far more bulk to dissipate heat).
If you are using Yorkshire fittings (with internal solder rings), the solder will visibly flow. keep some heat on both the pipe and fitting for a few seconds to ensure it's flowed all the way round and the full depth of the fitting socket.
With plain fittings, start trying to feed solder in at the junction of the pipe and fitting soon after the flux has boiled. Keep touching the solder to the pipe to check the temperature (with the flame away from it) every so often until the solder starts to flow. Alternate heat and feeding solder until there is a full ring of solder around the pipe. You only need to feed it at one spot for each joint, it should flow around by capillary action.
With either type of joint, once the solder has fully flowed, take the heat away and let it cool.
Be careful not to disturb the joint until it's cooled for a few minutes, or the solder layer can be disturbed and give a 'dry joint' which will be weaker and possibly leak.
When cooled , remove any traces of flux with a damp cloth, to prevent the pipe turning green.