You can use either baby spinach or mature spinach. I recommend trimming mature spinach stems before cooking. Spinach shrinks considerably when cooked, so keep that in mind. Feel free to double this recipe if needed.
Salt, pepper, red pepper flakes, and sugar all comes down to personal preference. I recommend tasting as you go. Start small and add as needed when it comes to seasonings.
The sugar called for in the ingredients simply helps balance out the bitterness. You'll really only need a pinch, but tweak it after tasting to suit your tastebuds.