For the slip knot, you'll want to lay your cord down and cut the thread that you've been wrapping around your cord, leaving a tail.
Put this loop over your old thread's tail that is on your cord, then pull both sides of the string below your slip knot to tighten, You'll hear an actual pop when the knot is secure.
For the magic knot, you'll want to lay your cord down and cut the thread that you've been wrapping around your cord, leaving a tail about 4-6 inches long.
You'll take your new thread, and tie a double knot around the old thread's tail close to your cord, then trim off the small tail of the knot.