for consistently good soldering results, you need:
- a good (e.g. with accurate temperature control) soldering iron
- good solder (e.g. 60/40 or 63/37 rosin core)
- tip cleaning tool (brass wool is great)
- a separate flux source (tacky flux works well)
- isopropyl alcohol and a (tooth) brush
Treat your soldering iron well, e.g. clean the tip, dont let it hot when not in use. Its very important that the tip is smooth and shiny, not bumpy, dull or rainbow coloured.
If you are soldering to previously use solder pads, clean them properly before you start. Remove wire, pins, and as much solder as you can, by holding you clean, freshly tinned soldering tip to the old solder. Then clean with alcohol and brush.
Pre-tin both sides. Make sure the solder looks smooth and shiny on both ends. This may take several seconds and maybe a few tries to get it just right on both sides.
If you have too much flux on the board/wire after pre-tinning, remove with alcohol/brush.
Then get both parts touch each other, and the soldering iron touch both.
The two solder blobs should melt into one, and this final step should only take 1-2 seconds.