1) Measure each cell voltage to ensure they are all within .2 V
2) Hold the cells in parallel (contact + to +, - to -) for about a minute to ensure they are balanced (the cells exchange charge until they balance perfectly).
3) Glue the cells together with 3M77 or contact cement with + opposite - so that the contacts are far apart.
4) Cut and glue in a strip of balsa in the trough between the contacts and top of the battery.
5) Trim the contacts to to the length of the space between the cells.
6) Tin the contacts
7) Solder one + to - joint on top of the balsa
8) bend the battery contacts back and solder on the connector.
9) wrap the connector contacts with tape for strain relief
10) cover with tape or heat shrink.
-Darren