Asks about the method or means of doing something ("how" or "in what way"). Different from どうして, which asks for reason.
このアプリはどうやって使うの？
How do you use this app?
その問題をどうやって解決したか教えてください。
Please tell me how you solved that problem.
留学ビザはどうやって取ればいいですか？
How should I obtain a student visa?