Wash the forest fruits thoroughly and remove any stems or leaves. If using larger fruits like strawberries, cut them into smaller pieces.

In a large pot, combine the forest fruits and sugar. Let the mixture sit for about 30 minutes, allowing the sugar to draw out the natural juices from the fruit.

Add the lemon juice to the fruit and sugar mixture. Cook over medium heat, stirring frequently until the sugar has completely dissolved.

Gradually stir in the pectin, ensuring it is well combined and free of clumps. Increase the heat and bring the mixture to a rolling boil. Boil for 5 minutes, stirring constantly to prevent scorching.

Test the jam for the desired set by performing a cold plate test: place a small spoonful of jam on a cold plate and let it cool for a minute. If it forms a skin and wrinkles when pushed, it's ready. If not, continue cooking for a few more minutes and test again.

Carefully ladle the hot jam into sterilized jars, leaving a 1/4-inch headspace. Wipe the rims clean, place the lids and bands on, and process in a boiling water bath for 10 minutes to seal.

Remove the jars from the water bath and let them cool completely. Check the seals, then store in a cool, dark place for up to one year. Refrigerate after opening.