Put the heat shrink tubing on and slide it away from the joint.
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Can’t tell you how many times I have had to cut my work apart because I forgot to put the heat shrink on first. Also put it far enough away so that when the heat migrates up the wires, it doesn’t shrink the heat shrink prematurely.
Also, don’t go crazy with the solder. If you apply too much solder to the joint, it will wick further up the wires than desired, causing a stress riser which could cause the wires to fatigue and crack next to the rigid solder joint. Only use enough solder to make a sound joint.
Next, don’t get the copper too hot. The solder will fall off in a glob instead of flowing. Flux, used to clean the copper, will burn and contaminate the solder joint. At that point, it is best to let it cool, clean it, and start over.
