To determine whether or not the jelly is done cooking, use a candy thermometer and the "sheeting method," described below. Don't use overripe fruit for jelly, as it may not set up well.
Combine fruit and water in a saucepan. Bring to a boil, reduce to a simmer, and cook, partially covered, mashing occasionally with a potato masher, until fruit is very soft, 10 to 15 minutes. Transfer mixture to a fine sieve set over a heatproof bowl; let drain without pressing on fruit, 4 hours. Strain again through sieve lined with damp cheesecloth. Measure juice; you will have 3 to 4 cups.
In a large heavy-bottomed pot, bring juice to a boil. Add 3/4 cup sugar for each cup of juice. Add lemon juice and 1/4 teaspoon salt. Return to a boil and cook, stirring frequently, 8 to 12 minutes. To test if jelly is done, dip a large metal spoon in, lift it horizontally above pot, and let mixture drip back in. Jelly is done when mixture has thickened slightly and drops of it slide together off spoon in a sheet. (Temperature should register 221 degrees on a candy thermometer.) Skim foam from top.
Ladle jelly into clean containers, leaving 3/4 inch of headroom. Let cool completely. Cover, label, and refrigerate up to 1 month, or freeze up to 1 year.