VP5500/serial howto
Aus LaborWiki
Open the battery cover and disconnect the battery. Then solder 3 wires to the RX, TX and GND pins:
Break of a tiny bit of the back cover. Try to not break the center pin of the back cover since this gives mechanical support for the screw you need to reattach later on.
After re-assembling all the stuff, it should look somewhat similar to this...