To set a jam or jelly or marmalade you need pectin. Some fruits are naturally very high in pectin but for strawberries you need to add pectin. You can make your own pectin from apples or you can buy pectin in the shops in the baking aisle ( look for Certo ). I cheat & use jam sugar.
Jam sugar has added pectin so it makes it very quick to make a quick tasy jam.
You need to sterilise the jars & lids. I do this by running them through a hot dishwasher. I then put them on a baking tray and put them in a low oven to dry them & make sure they are warm when you put the jam in.
You need a couple of punnets of strawberries ( 900g ) & 1kg of jam sugar. Prepare the fruit by taking the stalk off, then wash. I don't like whole fruit in my strawberry jam so I put them all in the blender and chop quickly till I have a lumpy blend.
Put the strawberries and sugar in a large pan and heat on a gentle heat till all of the sugar has dissolved. Don't boil until all the sugar has dissolved or you'll end up with sugar crystals in the jam.
Then bring to the boil, adding a knob of butter ( this reduces the amount of scum that forms on top of the jam ). When boiling, boil for exactly 4 minutes.
As soon as the timer pings, take the pan off the heat & skim off any froth ( scum ) on top.
Quickly decant into warm jars ( it will make about 4 x 1lb jars ), screw on the lids and you're finished.