how; in what way; by what means
Asks about the method or manner of doing something.
どうやって is the standard way to ask 'how' when you want to know the method or process. It is a combination of どう (how) and やって (the te-form of やる, meaning 'to do'), but it is fixed as a single question word. You can use it with any verb, not just やる: どうやって行きますか (how do you go?), どうやって作りますか (how do you make it?).
It is used in both casual and polite speech, but in polite questions you add か or the polite form of the verb. In casual speech, you can simply say どうやって行く? with rising intonation. Unlike なぜ (why) or なに (what), どうやって specifically asks for a step-by-step method or a tool, not for a reason.