By using bad copy on Google AdWords.
Lack of consistency is an absolute killer of conversions, no matter what the context.
If you’re advertising free shipping and you send your prospect to a landing page that doesn’t even mention it, you’ve failed, no matter how good the copy on that page might be.
Don’t use dynamic keyword insertion
If you don’t know what Dynamic Keyword Insertion (DKI) is, it’s a technology that allows Google to insert the exact keywords the customer is searching for in your ad copy.
You’re also going to run into issues with relevance, because it’s nearly impossible to get 20 keywords that will hang together coherently with good ad copy.