I have soldered hundreds of cells, and have picked up a few rules for safety, reliability and ease.
Not the final "best method", but fair, basic guidelines.
If pulled from notebook packs, they are pre-tabbed, soldering is simple.
If bare cells, more difficult.
1. Make sure that solder points are clean, abrasion, with emery cloth, or dremel tool, works great. Typically, there is a "glaze" or plating present, scraping with a sharp knife works, also.
2. Use soldering paste - flux, a slight dab is plenty. (I use a Q-tip)
3. Apply small bit of solder to soldering iron, (25w minimum, higher recommended), allow to attain full temp.
4. Apply to abraded fluxed point for 3 seconds, remove iron and confirm solder bead applied. If failed, allow to cool, then reflux and retry with slightly longer application.
5. Connect cells. I just began using a pre-tinned copper braid, (Tinned Copper Braid
). It works wonderfully. Pre-flux and apply small bead of solder to braid, or wire, at the connection point.
6. Apply braid to cell, apply solder to iron, pause for temp raise, push iron on top of braid till it collapses onto cell.
7. Remove iron, braid must cool for several second, before setting.(hold in place with toothpick, pencil etc.)
Method should be adjusted for variations, quality and wattage of soldering iron is the major variable.
A more powerful iron spends less contact time on battery and actually transfers less heat.
Practice can be done on "dead" cells.
"Practice makes perfect."