Connecting two wires together is a fundamental step in electronics. Whether you're using butt connectors, spade connectors, pigtail splicing, inline splicing, bus bars, or any of the myriad of other options available; the quality of this connection is often trivialized. When in reality, depending on your project, it could be a critical point of failure within a very complex system and could potentially lead to complicated repair efforts later on.

Coming from a marine electronics background there is a lot of concern for poor connections between wires and the build-up of corrosion in open joints, this is further complicated by the nature of the environment in which they are designed to operate (vibrations, saltwater, etc.). For these reasons I have always found the inline splice to be the strongest and most reliable connection for most of my applications.

I will walk you through my process from untouched wire to fully connected, soldered, and dolled-up.

Step 1: Required Tools

Wire - Any gauge really but in this example I used 18 AWG.
Heatshrink Tubing - In this example I used basic HST but in marine applications I normally try to use one with an added adhesive which helps to keep moisture/corrosion out.
Third Hand - This is optional of course and rarely available anywhere but on a work bench but incredibly helpful.
Wire Strippers - Formal strippers may not be necessary, any tool capable of cutting through the wire's insulation without cutting through the copper conductor will work.
Solder - There are far too many opinions on which solder to choose and the safety value of each. I'll leave you to research lead toxicity and such on your own; I'm just telling you that you need solder to properly execute this type of splice and that there are potential health concerns when using it without proper ventilation.
Soldering Station/Torch - Any will work, I have a really nice digital bench setup at the shop due to the variation of things we solder from tiny surface mount components to just connecting two pieces of wire. In my truck for on-the-spot connections, I use a sort of off the shelf propane torch.

In the end you only really need wire, solder, and soldering tools. Heatshrink tubing and proper wire strippers are a luxury that will make your project go smoother and end result more aesthetically pleasing.

Step 2: Stripping the Wire

How much insulation you'll need to strip away from the end of your wire is highly dependent on what gauge of wire you're using and what you have available. For the 18 AWG wire used in this example, I stripped away about an inch from each wires end.

When stripping the wire I generally make the incision with a wire stripper but remove the insulation with my fingers or pliers so that I can twist it as I remove it creating a much easier product to work with. Do not fret too much if the length of your copper is not identical on both sides, just get it "close enough for government work"; it can be trimmed up later on.

Step 3: Twisting & Soldering

With two stripped wires in-hand, the first thing you're going to want to do is to cut a piece of heat-shrink tubing about twice the length of the bare conductor on each wire. For example, in this instructable I stripped off an inch from each cable so I made my heat-shrink tubing about two inches long. It's very easy to reduce it's length afterwards, adding more after you solder is an unlikely task... Slip the heat-shrink tubing over one of the wires and slide it down and out of the way for now.

Now simply cross the wires over each other in an "X" meeting about halfway down the bare conductor as illustrated above. From this point just twist the two wires together as neatly and tightly as you can. Some people will argue that x amount of wraps are necessary and so on, I'd say at minimum of 3-4 and you're good.

Place some solder (tin) on your soldering iron and touch it to the mid-point of the twisted pair of wires. Once you've applied the tip of the tinned soldering iron to the center of the twisted pair, apply your additional solder to the bare conductor without touching the soldering iron until it melts into the copper creating a strong bond.

Once you're comfortable with the integrity of the bond created between the two wires, snip away any stray copper strands so you have a smooth surface to work with in your clean-up stage.

Note: It is important to understand that when soldering this splice (and in most soldering applications) you're goal is to heat the copper to the point where the solder melts into it and not to melt the solder onto the conductor.

Step 4: Wrapping It Up

You've created the in-line splice and snipped away any stray strands, so now it's time to make it pretty. Just slide your heat-shrink tubing back over the bare copper so that the tubing extends onto/over the wires insulation on both sides evenly.

Apply indirect heat to the tubing using a lighter, small torch, hair dryer, or whatever you have on-hand until it hugs the bare copper tight.

That's it, you're done! I've been tempted for a long time to test the difference in strength between the untouched wire and that of the in-line splice with my money on the splice being substantially stronger. Perhaps that's another instructable to come...

Disclaimer: This is my first instructable and I look forward to adding more in the near future. Having said that, I am very open to any constructive criticism. Thank you for reading and I hope it helps someone.