Pour the crushed strawberries with all the juices into a large pot. Measure the sugar into a separate bowl and set aside. Stir the package of pectin into the strawberries and add the butter.

Stirring frequently, bring the strawberry mixture to a full rolling boil, and then add the sugar. Stir to fully incorporate.

Return the mixture to a full, rolling boil and boil for exactly 1 minute, stirring constantly.

Remove the pot from the heat and skim any foam from the surface.

Ladle quickly into clean jars. Wipe the rims of the jars with a clean cloth, place the lids on, and tighten the bands to 'fingertip tight'.

Place the jars in a canner and process for 10 minutes. Ensure the water covers the jars by an inch or two.

Carefully remove the jars from the canner. Cool completely before storing.

Check that all lids are sealed properly before storing. If the lid springs back when pressed, store in the refrigerator.