It should be a fairly straightforward operation.
Personally, I lubricate each new cable first, and then assemble the three cables into the junction box, then put the cable assembly in position and fit the slides and fit the upper cable to the twist grip.
Removing/replacing the carb bodies is much easier if a cut-down allen key/hex key is used, as there's not much space to get at the two cap head screws between the carbs.