For 'lectric soldering, I rely on flux cored solder. Olde worlde irons used to need tinning, which involved much attacking with a file, then getting it hot and dabbing with cored solder in the active area, then a quick wipe to reveal a silvery area. Modern irons usually have iron coated tips and must never be filed for tinning, as this effectively destroys them. Get hot, dab of cored solder and wipe to leave a surface of liquid solder on the tip which ensures maximum heat transfer.
If the wire is heavy and untinned, the flux and solder treatment helps, but if the wire is pre-tinned, all that is needed is a quick dab to ensure that the tinned surface has had any layer of oxide removed.
When the mechanical join is made (posh way of saying the wire is wrapped round the tag/other wire) I dab the end of the tip to ensure liquid solder there, touch the joint and apply solder. Take the iron away as soon as the solder runs, joints a good 'un.
The secrets are
1 Clean
2 Speed - the iron needs to be hot enogh and big enough so that it doesn't cool down too quickly when in contact with the workpiece.
3 Clean
4 If possible, avoid separate flux, use cored solder
5 Clean
After making the joint, wipe the tip so that it is Clean for the next one.
Stranded wire can be a problem if it has gained an oxide layer, as cleaning it is almost impossible to do reliably. An acid flux gives the problem of ensuring removal which is vital as thin wires are very easily eaten away, especially when electricty is involved due to electrolysis.
Have a look for someone selling leaded solder, the new muck seems to be a disaster waiting to happen.
There is a most instuctive thread ekswhere about structural soldering, but circuit soldering is a different world.