ok so today im going to show you how to build the simplest device on earth very basic soldering skills
now for the parts:
4AAA baterry holder
a case for the battery
a female USB cord
4AAA baterries(i personally use toshiba)
a fancy decal to put on top :)

Step 1:

ok first the parts :)

Step 2:

ok first you need to solder together the black(negative-) wire,to the negative leads on the battery holder then to the same for the red wire(positive+)and by doing so you conected the female usb cord to the battery holder :)

Step 3:

next put the device into your fancy case and plug in your phone(im using an S3 I9300) and it works just fine.In the end you should get around 5V of power which is quite enough for it to power your phone.Have fun!
Also if you do decide to build one could you send some pictures becouse im intereted in seeing how yours turned out :)