There will always be core keywords associated to your Guide subject, along with the targeted audience and niche. Be sure to test these keywords with Google Keyword Tool or a paid tool like MarketSamurai.

A good video to help you execute this step correctly is this one created by the team at Google:

The objective is to figure out what terms people are already using to try and find information related to your Guide's subject. Often, there will be multiple words or sentences - but each one is worth a different amount based on the amount of traffic it will drive.

While you don't need to be an expert in Adwords to see even the smallest benefits, using some of these advanced filters to can help you better refine your keyword search:

• Location and Languages: Make sure the country is set to your default (or largest) market and language (United States + English)
• Show ideas and statistics for: Make sure this is set to Desktop and Laptop devices - mobile ads are a different game
• Local Monthly Searches: This doesn't need a particular value at the moment. If you're a more advanced user or familiar with the search keyword landscape for your industry you might want to start at a certain traffic level
• Competition: Again, if you aren't familiar with the landscape, I'd leave this open just to see what's out there. Later on you'll want to target keywords with at least a "medium" competitive level as they'll typically be more proven and actually drive traffic.