Really want to know how to wrap electrical tape around TIGHTLY an in-line splice... Without three hands! U have to keep wire taught and wrap.
Why not use heat shrink tubing?
Another option is self amalgamating tape.
Or fix one end of the wire in a clamp, wrap a bit of tape round the splice and stick it to itself. Then pull the wire tight with one hand and wrap the tape round with the other.
Insulation tape tends not to be the best option for the job. As suggested above, heatshrink is good and self amalgamating tape is both insulating and waterproof. It also doesn't crack and fall off outdoor joints subject to UV. Another option I use is "fluffy" loom tape, which has just the right amount of non oozing adhesive and stops the noise when subject to any vibration.
I think MATTLOG is forgetting to twist the wires together. That will hold them just fine while you wrap with tape.
I am almost afraid to ask- what are you splicing?