I've got a spin on how I did mine.
I had a Pentair SAM light that was bad. I replaced it with an Intellibrite.
I cut the light off at the pool, right at the back of the light.
I had a square of Plywood I used as a work surface near the pool. I took a pair of vise grips and clamped the old cable to the plywood to hold it there.
I then stripped back the insulation about 2' on the old cable and on the new cable. Put Flux on the ends, spread the copper fibers on the old and new cables, and soldered them together with one of those Propane mini torches you get at Home Depot. You need to make pretty good solder connections because you will be pulling pretty good on them. Make the soldered wires close together again and tape with Good Fresh electrical tape a good taped joint that tapers the tape in about 3-4 inches on each side of the solder joint. You want it to slide well and not snag on anything.
I then had my wife in the pool to feed the cable thru and tell me when to stop. I got on the other end and used a pair of Vise grips to grab the cable and pull. Mine was a pretty stiff hard pull.
There's enough info here to be able to do the rest ofthe stuff here. Take pix of the connections beforehand with a Camera, if you are as absent minded as I am!
If you have any further questions, you can PM me a phone number, I'd be happy to answer questions.
My cable was a 50' pull.