Although I'm not familiar with your "whortleberries" since I live in the western U.S., I'm sure that it really doesn't make a lot of difference. We use a mixture of blackberries, strawberries and raspberries for our "preserves" as they're called here.
There's preserves, where the fruit is simply crushed and pectin and sugar added before processing, there's jely, where the juice of the fruit is extracted through cooking and there's our version of "jam" which is the juice with maybe half of the fruit added back.
So, for our preserves, never try to make to large a batch... no more than 6 cups of fruit at a time. (Our cup = 8 ounces by weight).
Mash the room temperature berries but don't make them into runny syrup. Take two packages of "No Sugar Pectin" (it cuts down on the required sugar but doesn't eliminate it). Mix one package plus 2 additional tablespoons of pectin from the other package (dry pectin works best), with 1 cup of granulated sugar.
Place berries in a heavy (preferably cast iron pan) and stir in pectin/sugar mix... bring to a slow boil and add 3 additional cups of sugar (for a total of 4) and stir just often enough to keep it from sticking. When it comes to a good roiling boil time it for 1 minute... remove from heat and add 1/4 cup lemon juice. This isn't always required, but we like to assure some acidity for better preservation.
During this process you should have been boiling your jars (but not the lids) for sterilization.
Now, take a large metal spoon, dip it in ice water and coat it with a layer of the berry/sugar/pectin mix and let it cool to room temperature... If you turn it on edge, it should "sheet" off, but not run freely. If it doesn't, brin it to a boil again, with another two tablespoons of pectin...
Place the mixture in the jars leaving about 1/2 inch headroom, put the lids on after cleaning the tops of the jars with a clean, wet cloth (place the lids just momentarily in hot water to soften the sealing edge), screw on the rings finger tight, and place in a pot of boiling water to cover the tops of the jars, Boil for 10 minutes to assure sterilization and sealing... remove and let cool... et, voila!