The MAKE blog shows you how to splice two wires to NASA standards using a method known as the Western Union or Lineman’s Splice.

Here’s an excerpt from MAKE:

1. The conductors shall be pre-tinned.
2. There shall be at least 3 turns around each conductor and the wraps shall be tight with no gaps between adjacent turns.
3. The wraps shall not overlap and the ends of the wrap shall be trimmed flush prior to soldering to prevent protruding ends.
4. Conductors shall not overlap the insulation of the other wire.

Illustration by: NASA