Remove any stems from your cherries. To pit the cherries, place them over the open mouth of a clean, empty glass bottle (e.g. a beer bottle) facing upwards. Use a chopstick to poke through the cherry so that the pit pops out straight into the bottle. Repeat with the rest of the cherries - you should end up with roughly 1kg of cherries.
Place the pitted cherries into a large pot with the sugar and lemon juice. Place over a low heat and stir occasionally until it starts to bubble. As you start to see juice coming out of the cherries, turn the heat up. Keep stirring occasionally until the jam just starts to simmer then turn the heat down to low and leave to simmer. Let it simmer for 20-30 minutes until it has thickened and will coat a wooden spoon with a jammy layer.
You can test if the jam is done by placing a small plate into your freezer for 5 minutes so it gets nice and cold. Scoop a spoonful of jam onto the plate. Let it sit there for a minute to cool then push with a finger - if the jam wrinkles and leaves a clear line it's done, if it is still liquidy it needs to be cooked for longer.
Remove from the heat and stir in the almonds and almond extract. Divide between sterilised glass jars and leave to cool. Store at cool place for up to 3 months.