Pick over the berries and remove any stems or spoiled fruit. Slightly underripe fruit is fine, as it contains more pectin and will help the berry jam set.

Mash the berries and measure them after you've mashed them. For every cup of mashed berries, add 2/3 cup sugar.

Prepare a water bath canner (if canning), and prepare jam jars.

Place the berries and sugar in a jam pot and bring the mixture to a boil over medium-high heat. Cook time will vary based on the type of berry, and high pectin berries (gooseberries, blackberries) will set faster than lower pectin fruits (strawberries). The average cook time is about 10-15 minutes, but closer to 5 for high pectin fruits and closer to 30 for low pectin fruits.

As the jam begins to thicken, test for set on a plate that's been placed in the freezer. Alternately, use an instant-read thermometer. Berry jam reaches it's set point at 220 degrees F (or 104 C) at sea level. Subtract 1 degree for every 500 feet in elevation. For example, at 1000 feet in elevation, the jam should finish at around 218 F.

When the jam reaches its set point, remove the pot from the heat and ladle the jam into prepared jars leaving 1/4 inch headspace.

Seal with 2 part lids.

If canning, process the jars in a water bath canner for 10 minutes (or 15 minutes if above 6,000 feet in elevation). Remove the jars to cool on a towel on the counter and check seals after 12 to 24 hours. Store any unsealed jars in the refrigerator for immediate use.

This jam will keep for several weeks in the refrigerator or up to 6 months in the freezer. Properly canned and sealed jars will maintain peak quality for 18 months on the pantry shelf. Refrigerate after opening.