To make a charger you don't need to know about USB beyond pinout, Vcc=5V and Imax. (500mA nominal). Data pins remain unconnected. Since Nokia phones refuse to charge with Vin beyond some threshold (about 3.X volts, don't remember exact value) you need a DC-DC converter with a current limiter to avoid overload USB port. That limitation will make charge probably slower than it would be with a std charger with greater current limit. Note that battery charge is managed by internal phone electronics, then don't worry beyond to build power supply.
I hope this can be helpful.
Cheers ;-)