expression, adverb
how; in what way; I wonder how
Emphatic expression used when the speaker is puzzled or frustrated about how something happened or should be done. Often implies 'I have no idea how' or 'no matter how I think about it'.
See also: 如何 (どう)
どこをどう探しても見つからない。
No matter where or how I search, I can't find it.
どこをどう間違えたのか、さっぱりわからない。
I have no idea how I messed up.
どうやって asks 'by what means', whereas どこをどう conveys a stronger sense of confusion or helplessness about the way something is done.
Combination of どこ (where) + を (object marker) + どう (how). The phrase literally means 'where and how', but is used idiomatically to express confusion about the method or process.