Rachel gave some basic recipe guidelines. For example, macerating the fruit with the sugar in advance (24 hours or more in advance) is recommended as standard practice for best results. The sugar-to-fruit ratio ranges from 1:1 to .3:1; the amount of lemon varies widely, and is best determined by taste testing (before cooking), adding a tablespoon or two at a time.
I'm anxious to try other fruits, but I had some peaches on hand that I needed to use up anyway, so that's what I happened to make as my first attempt...
3 peaches, peeled and sliced
2/3 C sugar
(I think I'd reduce the sugar a little next time)
about 4 Tbls lemon juice
Preparation: Combine peaches and sugar; refrigerate 24 hours, stirring occasionally. Macerating the fruit this way draws out the liquid out.
Add lemon (a little at a time, taste test as you go until the fruit flavor really 'jumps forward').
Use a wide pan (to allow heat to escape and liquid to evaporate; Rachel recommends investing in a copper, sloping-side pan if you plan to do a lot of jam making; I have a 3.5 qt wide, deep sauce pan that I used).
Cooking: If there's a lot of liquid, you can start right off on high heat. First, bring it all up to temp (boiling), dissolving all the sugar if it's not already. At first, there's lots of liquid and it's relatively light in color. If foam develops, remove from heat to skim off the foam. (My peaches didn't produce much, but the berries Rachel used in the class did.) As it continues to cook, the volume will reduce; the color will darken; and the bubbles start to get smaller, shinier/glassier and form small 'wells'; and it begins to sound almost like a rustling. As it nears 'done-ness' it's thick enough that as you draw a spatula across the pan it will clearly leave a track before syrupy-thick, hot jam covers the bottom of the pot again. Much of the fruit is broken down by this point and the jam is more cohesive, rather than chunks of fruit in liquid. (More or less chunks is a personal preference, and something to think about when preparing and slicing the fruit to begin with.)
You can remove it from the heat, scoop out a small spoonful and put it in the freezer a few minutes -- just to bring it more quickly to room temperature -- to test the texture for a spreadable consistency.
Pour or spoon jam in to sterile jars; seal.
(Rachel sterilizes jars on a cookie sheet in a 250° oven rather than boiling them in water, but either method works.)