4. Wash the jars thoroughly and dry with a clean towel. heat the jars in a moderate oven for five minutes to sterilise. Next, fill up the jars, using a jam funnel or jug. To seal the jam, put a waxed disc over the jam immediately and cover with cellophane pot covers (the cellophane will shrink over the top of the jars to form an air tight seal). Secure with elastic bands and label the jars when cold.