I start by looking at other recipes in the same style, and doing a little research. "Brewing classic styles" is a great resource. Look for patterns... what are others doing most often, what are weird outliers, etc.
Then, on to a brewing calculator. I use Beersmith, but there are others available. Start adding and subtracting, and see how it affects original gravity, color, IBUS.
In general, I try to stay to a pound or less of specialty grains per 5 gallons (I break this rule on my porter, but on light colored beers, it's a good rule of thumb). A pack of yeast for 5 gallons up to 1.055 or so...
It does all depend. Look up some clones, or some of the NB kits, and look for patterns.