Generally you will need equal amounts (about 4lb)of fruit and sugar - raspberry and gooseberry are good, strawberry not so good - a bottle of pectin and a big, big saucepan.
Put the fruit into the saucepan and heat gently, allow the fruit to cook in its own juices until very soft. This takes about 20 minutes.
Add the sugar, stir to dissolve, then add the butter and pectin. Raise the heat and bring to the boil then continue to boil hard for about 20 minutes. You must get a really good rolling boil otherwise the jam won't set.
Put a small amount of the jam onto a plate, leave for a few seconds to cool, then push with your finger. If the jam is ready it will set, and wrinkle when you push it. When it's ready, let it cool for about 5 minutes, skimming off any scum with a metal spoon.
If you use the sugar jam, though, don't use the pectin.
You may find for fruit like raspberries you don't need to add the pectin anyway because there's quite a lot of pectin already in the fruit, but it won't hurt either way. You would need pectin for strawberries though.