One the questions that I am frequently asked by homebrewers is ìhow do you come up with new beer ideas? As a homebrewer, there are plenty of ways to find pre-published recipes. I started out by purchasing pre-assembled kits that mimicked a particular beer style. After this I ventured on the internet and read magazines to find published recipes to replicate. Finally, I began to take off the training wheels and experiment with my own recipe creations.
Let's get back to the question. Let me ask some different questions. How does an artist decide what to paint? or When does a songwriter decide to end the song? These should be rhetorical questions by nature and the answer is something akin to inspiration. The same can be said for a brewer. I'm inspired by all sorts of things: foods, delicatessens and candy, great commercial examples, or reading about a particular historical style of beer (that sometimes has become extinct).
I've developed my own combination of gut-feel and scientific numbers to develop a solid, brew-worthy recipe. There are a lot of magazines like Brew-Your-Own and Zymurgy and books like Designing Great Beers by Ray Daniels, and online resources like Craftbeer.com that can help you find guidelines for attempting this process. If I were to share with others my basic thought processes that I use to create a beer recipe they would be something like this:
Pick a style of beer or inspiration of several ones and combine them. Refer to BJCP.ORG to review the flavor guidelines for making such a style(s)
Pick the malts that you want to build the beer, paying attention to the % of each. Base malts can make up 100% of the malt bill, but some specialty grains should be no more than 10%. Balance these the way you'd like and double check what the estimated OG (original gravity) will be. OG determines the amount of potential alcohol, a higher OG usually results in higher alcohol.
Next pick the hop varieties and determine when you want to add them. Always use a bittering hop addition and depending on the style sometimes a flavor, whirlpool/aroma and/or dry-hop addition(s). Lastly, balance them to get the proper amount of IBU's (international bittering units) to meet your tastes or fit the style of beer you are brewing.
Finally, pick the yeast that is right for said style and decide how you want the yeast to behave (different variables like pitch-rate, temperature, and aging time will affect flavors)
This last yeast step can be a simple task or very complicated. You'll have to study many of the characteristics that the yeasts will lend a beer. Alternatively, if you are brewing a particular common style like Scottish Ale, then there are commercial varieties that are labeled as such. If you want quick and easy guidelines, most American strains will be clean (low levels of complex yeast flavors) and high attenuating (eats up more sugars than other yeasts and can handle making higher alcohols), English strains are more fruity, Lager strains are going to be more sulfury and clean, and hefeweizen or Belgian-style strains will be more phenolic and fruitier.
Good Luck and Prost!