Wash and hull the strawberries and remove any squishy or bruised pieces. Use a potato masher to mash the berries slightly (they will cook down further). Add to a large non reactive pot along with the sugar and mix together.
Bring to the boil over a medium/high heat, stirring occasionally. Once the jam has come to a rolling boil, clip a candy thermometer onto the side of the pot and continue to cook over a medium/high heat until the jam is thick, glossy and reaches setting point - 105°C/220°F - this took about 35 minutes for me (see notes if you don't have a thermometer)
Using a jar funnel, ladle the jam into sterilised jars, leaving ½cm (¼ inch) gap at the top. Use a clean cloth to wipe the rim of the jar to ensure it is clean and place a lid/seal on the jar and screw a ring/band until fingertip tight.
Process in a water bath for 10 minutes (see notes for details on water bath canning), remove from the water bath and leave on the bench overnight. In the morning, check the seals. Any that haven't sealed need to be stored in the fridge. Jars that have sealed can be stored at room temperature for several years.
How to test jam for a set without a thermometer:
Before you start cooking the jam, place a saucer in the freezer. When you think the jam has set, take the jam off the heat, place a spoonful of jam on the saucer and place back in the freezer for 3 minutes. When you push your finger through the jam, it should almost wrinkle and will feel set and that's when you know it is done.
By the look of the jam - it will become thick, glossy and much darker in colour. When you pull a spoon out of the jam, it shouldn't drip off but form thicker droplets that don't immediately run off the spoon.
Water bath method:Many people are happy to just bottle jam and not process the jars - I have bottled jam like this in the past and it has kept well for years! I now prefer to water bath all my preserves as a guarantee that they will keep well.To water bath - fill your largest stock pot with hot water and bring to a rolling boil. Place a rack or canning lids on the bottom of the pot (you don't want the jars directly on the bottom) and as you fill the jars, place them in the water bath. Bring it back to a rolling boil and then start the timer for 10 minutes. After 10 minutes is up, remove the jars from the water bath and place on a clean tea towel on the bench to cool.