The hard part is the hardware as you should really remove the battery from the equation to prevent catastrophic damage from constantly draining and charging an old worn-out battery. Unfortunately most older phones won't run off wall power without a battery inside so one method is to solder some wires attached to ~4V to the battery contacts in the phone to trick it into thinking there is a battery present and allow you to run the device off USB power directly. This method might vary from phone to phone.