All kinds of soft fruit can be used to make jam. If possible, use a book of jam recipes which will tell you exactly how much fruit, sugar and water to use for each different fruit. However, if you cannot find such recipes, here are the general principles of jam making.
Use ripe soft fruit, chopped into small pieces – for example, mango, cape gooseberries, raspberries, strawberries and guava all make good jam. Measure out the fruit in cups as you place it in a large pan to cook. Jam will bubble up a great deal as it cooks, so make sure you don’t use too small a pan.
For every 2 cups of fruit use just a 1/4 cup of water (less if the fruit is very juicy like pineapple, raspberries or strawberries). Cook the fruit until very soft (usually 15–20 minutes). Then add 1 cup of sugar for every 2 cups of fruit. Stir well and allow to boil for 15–20 minutes until it will set. Test for setting by dropping a small amount of jam onto a cool plate. After a few minutes, push it with your finger. If it wrinkles and forms a skin it is ready. If it does not, boil longer and try again. You may need to add more sugar.
Take very clean, dry jam jars. Before pouring the jam in each, wrap well with a cool, damp cloth to prevent the glass breaking. Fill and cover with a clean lid.
Remember the proportions – 2 cups of fruit : I cup of sugar : 1/4 cup water. Remember too that this is just an approximate estimate. Experiment with the fruit you have. Soft fruit needs little cooking. If it does not set well, use less water and more sugar. If it sets like a rock, use more water. Pineapple jam is very hard to set and does not keep well (but tastes good!) Make in small quantities. Oranges, lemons and other citrus fruit need much longer cooking and for every 2 cups of fruit add 1/2 cup water and 11/2 cups of sugar. Tie the citrus seeds into a piece of cotton and cook with the jam to improve setting.