Squash can be julienned with a knife, but an inexpensive julienner, like the one used in the pictures below,makes the job easier and the results better. Julienning is also a job that can also be accomplished with a mandolin, which is what many professional chefs use because they process such enormous quantities. For the small amounts required in everyday cooking, my handheld julienner is easier to use, costs a whole lot less than a decent mandolin–and is a lot easier to set up and clean.
Grasp the squash firmly in the palm of one hand . . .
. . . set the teeth of julienner firmly (but not too deeply) into squash . . .
. . . drag julienner down length of squash . . .
. . . and you get sweet, spaghetti-like strips.
Sian learns to rotate the squash after every two to three passes with the julienner to promote even cutting.