It saves some space in a case and sometimes it is easier to solder a cable to an Arduino than to plug and unplug it every time. Once tested it is the one plug less to worry about. - So let's see how it works.
Step 1: Peel Your USB-Cable
To start remove a board with of your cables outer insulation and pre apply some solder to every of wires.
Step 2: Add Stress Relief
To avoid an early tear-out of the cable, we wrap the cable through one of the mouting holes (which will not be available any more for screws, but who cares ;-)
Step 3: Sodler the Wire to the USB-Connector Pins
Now we can solder the pins of the USB-Cable to the bottom of the Arduino. Be sure to follow exactly the same scheme:
White -- Green
Red -- Black
Step 4: Test It
Before we finish the build, just test if you can upload the "blink" sketch as usually.
Step 5: Add Some Magic Isolations (aka Hot Glue)
To add some more stress resistance, add a big blob of hot glue.