In a nutshell, you reduce the length of one wire at neg so that the stripped soldering section is only able to contact insulated wire on the other line. This will work to reduce the risk of short before heat shrinks kick in.
Step 1: Cut Your Wires Unevenly
The purpose of this step is to leave enough insulation to prevent accidental short during work or due to poor taping.
Cut one first and trim extra from the wire you plan to solder it to.
In the picture I was hacking an old mac magsafe charger to a battery connection which I made for my macbook air(older version of magsafe is compatible with L shaped charger.) I trimmed way long from the mac neg end, which is the outer wire, and cut it to length for good solder connection, and then trimmed equal length off of the CCTV power pos wire so as to make them enevenly parallel.
Step 2: Solder Them In
While T shaped twist and solder will also work, I prefer a union solder so that they stay in a line shape, if all is done well, your heat shrunk connection should look almost factory made.
Step 3: Optional Liquid Taping or Just Tape the Solder Joints
Liquid electric tape is without a doubt better than regular tape, insulate the solder joints with tape or liquid tape.
Step 4: Heat Shrink the Wire
Depending on the length of wire and the diameter of the heat shrink of your choice you may need to feed heat shrink prior to soldering. For hacking macbook power cord, always solder the pos wire and heat shrink that first, then the neg, and then the whole wire. If you plan to only heat shrink the wire as a whole, that works too uneven insulation should provide some safety by itself, but safer is always better.
Step 5: Test Your Project
Soldering is a tricky thing, you actually want to pull reasonably on it a bit to make sure that the solder joints were made at decent temp. Cold solder joints will break, but once you get the hang of it you can come back and fix it. Always test the integrity of wire prior to use.
Step 6: Short Cable for Demonstration
This is only for demonstration, actual wire is longer, I made this for picture purposes, it's okay at this level, but could be better.