Once you've got your pie crust, you've got to fill it. And at this time of year, when the stores are full of the best of both summer and fall produce, that invariably means fresh fruit.
You can get as complicated or as simple as you like with a fruit filling. I made lots of pies by eye this summer, simply slicing up some fruit and tossing it with some sugar and some thickener (cornstarch, quick-cooking tapioca or flour).
My results were predictably unpredictable, but almost all were extremely edible and some were downright delicious (the one exception: do not use tapioca in a topless pie--it never softens and you're left with a tart decorated with hard little white bits).
Exactly how much sugar and thickener and which kind of thickener you use depend on the kind and quality of fruit--and on your personal taste.
I would rather have a pie that's a little runny than one that's straight-up firm. That means I tend to go easy on the thickener. This is especially true with fruit that is either slightly underripe or that is otherwise naturally full of pectin. Apple and blueberry pies, for example, take only a couple of teaspoons of thickener. Peaches and nectarines need more. So do raspberries, but curiously, blackberries are closer to blueberries in pectin content than to raspberries.
Different thickeners give different results as well. Cornstarch and tapioca will give you a clear, jelly-like filling. Flour is creamier but cloudy. Cornstarch and tapioca are also stronger than flour, so you'll need to use less. Some old guides use a ratio of half as much cornstarch or tapioca as flour, but I don't think there's quite that much of a difference. Maybe more like 1/4 to 1/3 less.
I also like a pie that tastes more of fruit than of sugar: If peaches are really ripe, only 1/4 cup of sweetener is necessary. On the other hand, if you've got really tart plums, it may take a full cup.
In general, you'll need about 5 or 6 cups of sliced fruit to fill a 9-inch pie. To sweeten that fruit, you'll need 1/2 to 3/4 cup of sugar. To thicken it, you'll need about 2 to 3 tablespoons of cornstarch or tapioca or 1/4 cup of flour.