Step 1: Unassemble the phone
Disassembling is not an easy task with this type of phone. You have to find these small plastic "noses" on the left and right side. Best I did to open the case was to slib between the small crack using my fingernails ... some broke during this. But finally the case was open and the parts lay in front of me.
Step 2: Putting power on the device
The red and the black cable are the ones that gat the power. The usb port is capable of giving you 5V on 1/2 amperes. This is enough for the phone to operate. Feed the cable through the hole in the plastic cover and solder it to the old battery contacts.