Once the sugar has dissolved slowly bring the pan to the boil and once at a rolling boil, time for 15 minutes, keep stirring, then take off the heat.
Sterilise jars by washing in hot soapy water (or take straight from dishwasher), fill with boiling water, empty and then place in oven for 20 minutes at 140°C where you leave them until the jam is ready. Washed lids should be sterilised with boiling water and then left to drain. To make a strainer if you don’t have a jelly bag etc, put a colander into a new pair of tights, tie the legs into a knot on the side and snip off the excess. You could also line a colander with clean net curtains, a tea towel or muslin. If your mint is bone dry it’s more inclined to float to the top so rinse your leaves, give a little shake to remove excess water and finely chop so that mint is damp when adding to the jelly.