Jam: the 'template' recipe

I wouldn't use the method below for strawberry jam; it requires a different method.

How to make jam

Ingredients:
some suitable fruit
granulated sugar
lemon juice, if needed
water

Notice there are no amounts.  That's because the amount of sugar depends on the amount of fruit.

You will also need
clean, warm jam jars with lids
a saucepan or a maslin pan depending on how much you're making
a jam funnel (optional but I wouldn't be without mine)
labels because you think you will remember but you don't, like things in the freezer - I use Avery labels (or similar and cheaper)

Method:
Clean and prepare the fruit (e.g. take out stones).  I usually leave the skin on unless it is very tough.

Warm the very clean jam jars in a cool oven.

Put the fruit in a saucepan with some water - not too much, just enough to prevent sticking.  Do NOT add sugar at this point.  Bring to a boil and gently simmer until the fruit is soft.  Stir occasionally.

Measure how much fruit mixture you have.  For every pint of fruit mixture use one lb of sugar.  (metric doesn't work here) If you want to use less sugar, do so, but the jam won't keep nearly so well.

Put the fruit and the sugar back in the saucepan (make sure it is large enough as the cooking jam bubbles up) together with lemon juice if needed/used and stir it all well to dissolve the sugar.  If your fruit mixture is warm, the sugar dissolves more quickly.  If it is cold, heat it all gently, stirring well, and don't let it boil until all the sugar has dissolved.

If using the wrinkle test for setting, put two saucers in the fridge or, if possible, the freezer.

Bring the jam mixture to a rolling boil, stirring well.  If you get scum, skim it off.  Boil the jam well for five minutes or so, stirring regularly, then remove from the heat and test for set (see previous entry).  If it has not reached setting point, reboil for another five minutes or so.

Once setting point is reached, skim off any remaining scum, if you want to, or add a knob of butter and stir it is which reduces the scum.  Leave the jam to stand for five minutes, then carefully (it is VERY hot) ladle into the hot jam jars.  I find using a funnel cuts down amazingly on stick mess.
Screw the lids on tightly.
Don't label until cold, not because the labels won't stick, they will, but they are the very devil to remove afterwards if they go onto hot jars!  Bitter experience, believe me!

Enjoy your jam!