I am getting a message on my coffee maker that it should be descaled. However, I can not figure out how to do it without the manual, which is missing.
Citric acid isn't very effective, it's even weaker than vinegar. Sage (Breville's trade mark in the UK) advertises with a descaling powder containing sulfamic acid (see someone who figured this out on a forum), which is a much stronger acid.
The weaker acids may be fine when they sit longer, for example when you're descaling the bottom of a kettle where the solution is in contact with the bottom for the duration of the descaling process. In more complicated espresso machines, you have tubes (metal or plastic) which may also have some deposits. Depending on the descaling process, the solution doesn't stay in those tubes very long, so you need to have a stronger solution that can clean the deposits there in that limited time window.
There are also aftermarket products that contain this acid (in power or liquid form). On my local Amazon, the powder goes for about 10US$ per kg (2.2 lbs). You need to make sure your machine can handle these, I think the ones with a stainless steel boiler can (at least the more expensive Breville machines are stainless steel, not sure about the cheaper ones).
Once you've settled on using this acid, you need to figure out dosage. I'd recommend following the dosage of the aftermarket solution or Breville's own powder and following the descaling instructions that came with your machine (or by looking up the manual online).
Citric acid's acid-to-water ratio is about one part acid to 20 parts water. Vinegar's acid-to-water ratio differs by recipe, with most calling for equal parts water and vinegar or less; use the same ratio for lemon juice recipes.
Easy DIY article! You can make your own equally effective homemade coffee descaling cleaner for any coffee maker with scaling build-up at a fraction of the cost.