「どうして」 here means the same thing as 「どうやって」("in what manner", "using what method", etc.).
Thus, as the given translation says, the sentence means:
"How should one do [go about (doing)] this?"
If you want a really raw, literal translation, it would be:
"As for this, in what manner, does one do?"
which is why the TL your falshcards give you is very good.
Why not simply どうする?
Because that would mean a completely different thing.
"What should we/you/one do with this?"
The 「どう」 in 「どうする」 means "what" rather than "how", which is a common mistake among Japanese-learners.
Finally, not to confuse you, the original sentence in question 「これはどうしてするのですか。」 can also mean "Why do you/we (have to) do this?" because 「どうして」 can also mean 「なぜ」. As usual, which one the sentence means depends entirely on the context.