どう means 'how'. どうでしたか？ - how was it? どんな means 'what kind of'. どんなくだものがすきですか？- what kind of fruit do you like?

For example: どんなかもくがすきですか。- Which/what subject do you like? これはどうですか。- How is this? ("this"refers to an object)

どう is how, ie., how something is and どんな is 'which' In context, どう is used when asked why, or how something is done/ why (usually). どんな can be used when someone is asking another person 'which' (depending on the topic) one/thing (usually) they prefer. But basically, they are both very different. Hope this helped! :)