どうやって？ - you are asking how. For instance, "どうやってピアノを弾くの？" means "How to play the piano ?" どう？ - you are asking for an opinion.(Same meaning as "What do you think?".) The word also has combinations with some letters, each has a different meaning. However, you don't need to think about so much, It's ok just remember as a phrase. For instance, "どうして？" - you are asking why. "どうする？" - you are asking about some plan. "どうやって？" - you are asking how.