Always use fresh and good quality fruit for jams. The best choice is seasonal fruit, beside the fact that they are tastiest, you can get a pretty good price for it as well.
The easiest way to prepare jars is to wash them in a dishwasher, which is what I normally do.
Wrap star anise and cinnamon stick in a cotton cloth and tie with cotton string, or just cook them as they are if you don’t mind them being part of the jam. (If you decide to use cotton wrap don’t forget to take it out at the end of the process). Wash rhubarb carefully, peel and chop into 2 cm pieces. Wash and hull strawberries as well.
In a large pot add rhubarb, strawberries, apple and spices. Cook on a high temperature until it boils, then reduce heat and cook for 30 min. Add sugar and vanilla pod while cooking on a low heat for another 1 1/2 hour or until you reach desired thickness, stirring occasionally.
I like jam to be quite thick and 2 hours is just what I need to reach that thickness but as always I embrace creativity, so make your own decision and jam. Once cooled pour jam into a clean jars and seal.