Step 1: Get the stuff!!
One standard lenghth cat. 5 ethernet cable
The stock cat. 5 cable of your desired length
Pin nose pliers
knife of some sort.
Step 2: Cut the standard one in half cleanly
Then cut off about an inch of the insulation off both cut ends.
Step 3: Clean off the ends
Just enough to twist each together
Do the same to your stock wire
Step 4: Now for the splicing
There is a chart of the colors at the bottom of this site.
There is a couple wires that are hard to tell what they are.
When you twist one together use electrical tape to shut it off or you could fry something maybe....
Step 5: After doing each side
Step 6: Test it
I am not responsible for any shorts that probably has to do with matching the colors wrong...