This question was already asked a couple of times, here and here for example, although no answer was accepted.
Both des and les can be used in your sentences.
Comment xxxx des légumes ?
How to xxxx vegetables ? (any kind of them)
Comment xxxx les légumes ?
How to xxxx the vegetables ?
with xxxx being one of the verbs of your examples (confire, braiser, cuisiner, blanchir, préparer...).
Here the meaning might be identical to the previous form, when the vegetables is representing "the vegetables in general" but it might also means "the vegetables that we are talking about, the vegetables I need to cook". The context should help figuring out which meaning is intended.
In the last sentence:
Je prépare des légumes frais pour cette salade
You tell you are using some fresh vegetables to make this salad while in:
Je prépare les légumes frais pour cette salade
you tell you use the fresh vegetables (all the ones that are already here) to make the salad.