For my own garden, I like to start with the least predictable of those variables—the weather.
First I look up my average frost dates by zipcode. There are different places to do this, and they may each give slightly different results—depending on how many and which years they use to calculate the averages—so I go to a few websites and see where they agree. My personal favorite site is Dave’s Garden, which has the most specific information.
If that’s too complicated, you can use the average frost dates by zone printed in the calendar. This isn’t quite as accurate, but it’s enough to be useful. (Frost dates are always a guess, never a promise.)
If you really want to fine tune things here, you can look at a long range forecast such as the one available from the Old Farmer’s Almanac book or website.
This is also a good time to consider your microclimate—are you on a northern or southern slope? Near pine forests or parking lots? All of these things can cool or warm the soil—or even create a tiny microclimate within your larger garden or farm.
Now that I know where in the wheel of the year I’ll be planting (usually February or March until May or June, with another round later on in the summer), it’s time to look at specific dates and their phases and signs. In my research for this calendar I found people who give more creedence to phase and others who favor the signs. I prefer to take it all into account, but of course the optimal combination doesn’t always occur at the correct time for planting.