1. the suggestion to download the SRAM instructions doesn't help. They just show how to thread a new cable thru.
2. I'll try to clarify the disassembly. Once you've removed the shifter, you'll see the white (?) plastic retainers within the twist-grip. They are flexible (somewhat). Bend one of them a bit inward w/a finger, and you'll see how the 4 hold the twist-grip in place via the lip on their outer end. If all 4 are bent at once, the twist-grip can be slid off. Find 4 objects to wedge between these and the grip simultaneously. I use 4 small allen wrenches, but 4 nails or 4 toothpicks, etc., would likely work. Don't slide the grip off carelessly! Slide it over those lip retainers just a bit (you can remove the toothpicks, etc., now), and observe the internals of the shifter, especially the metal springsteel that provides the indexing/snap of the shifter. Note the structure & arrangement of parts. Once you've opened the shifter adequately, the grey triangular cover can be gently pried off. Don't forget to replace this cover first as you reassemble.

It took me about as long to type this as it does to replace the cable. I've done it a few times now, and it does get easier. A pair of surgical needle holders or needle-nose pliers help w/the spring-steel, which will likely pop out at some point. Don't lose it! I've yet to wear out these MRX cheapies after many thousands of miles and several cables. Who says quality doesn't come cheap?!