Put the strawberries with sugar (keep 1 tbsp of sugar for later) and lemon juice into a shallow pan, bring to boil, lower the heat to medium and cook for 1 h.
Mix the pectin with 1 tbsp sugar, add to the cooked strawberries, stir and cook for further 5 minutes.
Pour the jam into sterilized jars*, cover with lids, seal and put all the jars upside down on a towel, then cover them with the towel tightly.
Keep the jars covered for a few hours. Afterwards, you can keep them in a cool place for up to a year.
Notes
* There are many ways of sterilizing jars, but I have been successfully using this one:
wash jars and lids very well,
put them in a big bowl,
a minute or so before you are ready to put your jam in jars, pour boiling water into the bowl with jars,
after at least 30 seconds gently take one jar from the bowl and pour in the jam,
the jars will be very hot, so you have to use an oven glove to hold them.