you need a 9 V battery
use lm7805 which is 9V-5V voltage regulator
in input,attach the positive terminal of 9 V battery
on the output side you have 5V.
ground pin 2
assuming you will be using a usb data cable for charging your phone-
you need a usb cable with female end and the other end is cut out. just as in this photo-
this is how the inside of usb cable looks
now connect the the output of lm7805 to pin1 of usb (5V )
ground pin 4
and between pin 2 and pin 3 attach a 200 ohm resistor. this is done to trick your phone into thinking that it has been connected to a normal charger.
also this value of resistor is for iphone. you will have to check out the value for other phones. may or may not be the same. some phones work even by simply grounding pin 2 and 3. for some phones you need 100-200 ohm resistor. check by trail and error basis
now simply connect the male end of your usb data cable into the female end of this usb cable and your portable charger is ready
you may also add a switch if you want