Please read my post on building Solar Panels from new (untabbed) Cells before this post on scrap cells. A lot of the info is applicable to both.
My major problem was finding the right temp to solder at.
270 to 280 seems to work best.
For the front silver strip (centre bus) I scratched a 1cm area with my pocket knife until it was really shiny (being careful not to go right through) then applied a little resin flux to the area.
I next applied the tip of the soldering iron for a few seconds before applying the solder to the bus at the tip of the iron. Overheating at this point will prevent the solder taking as much as underheating will.
I found the solder balled at first and stuck to the tip, then I lightly rubbed the tip of the iron back and forwards along the bus till it felt a bit "sticky".
That "sticky" feel indicates the solder has taken.
You only need a line of 1/2cm or so of solder (opinions vary from 1cm to the whole bus).
I did no more to the front at this stage.
For the backs of the cells I applied resin flux to a small area in line with the front silver strip (bus) but near the opposite edge to where I put the solder on the front bus.
Temperature is more important here.
No bus to solder to means you have to make one.
Apply resin flux to the area you want to create the bus on. You only need one.
Heat the area with the iron for 3 seconds then apply a small amount of solder to the tip of the iron.
When the cell is the right temp the solder will spread easily and obviously "take" to the surface (If the cell/iron is too hot the solder will spread but not take If it's too cold the solder won't spread).
I then put the tab in place. Holding it gently with the tip of the iron I heat it and the solder on the cell.
I then apply a little more solder to the tip of the iron.
This causes the solder under the tab to melt giving a nice bond.
Once again heat is important.
The pics show a little "budgie poo" of solder on the front bus, the tab attached to the back of the cell and two cells ready to join.
Joining them works just the same as tabbing the back....put the end of the tab over the solder on the front bus, heat, apply a little solder and jobs done as long as you got a nice "take" when you soldered the front.
If you struggle using tab try with a little fine copper wire instead of the tab.....maybe your iron isn't giving you what you need for the heavier tab.
Another major helpful thing I did was to mount a little "extractor" fan (off an old P2 processor) on my workboard.
A lot of solders per panel meant a lot of fumes were going up my nose.
The fan means I can breathe and solder at the same time....a nice feeling.
I also use a magnifier lamp...I'm an old bugger and it helps.