I have made 6 lt bottles plus 10 1/2 lt bottles - it is so easy!
Get the kilner style jars with the metal lids (not frnch kilner jars with the flippy spring/glass lids)liefheit and ...Ive got Itlatian named ones - sorry without checking couldn't say.
Wash in hot soapy water, rinse and sterilise in whichever way you prefer, pack the cooled jars with good quality, firm washed plums - really pack them in or you will be left with a big gap at the bottom after. Preheat oven to 140 for fan 150 for conventional
Make syrup - 1 litre water and 1lb sugar, heat slowly until sugar dissolved, bring to boil and then simmer for 2 mins. I add a few cloves a stick of cinnamon and a star ainse - however fragrant you like it I suppose.
Cool for a few mins and then fill jars to about .5-1cm below top, covering fruit. put on lids, loosen off a 1/2-1/4 turn.
Put several layers of newspaper in a stury roasting tin, stand jars in it an inch or so apart - DEF not touching. Bake for 35 (for small jars) or 45 mins (for big). Remove from oven, tighten lids straight away, wipe with a really HOT damp cloth. Leave to cool and the vacuum bit (bump on the lid) will pop inwards.
Good luck. And thanks to another Nmer who found this on the net - it works very well.