Before you start
Breville machines (sold as Sage in the UK and Europe) share a similar descale procedure across the Bambino, Barista Express and Barista Pro families. The button combinations differ by model, though, so keep your manual nearby for the exact sequence.
You'll need a Breville-compatible descaling solution, fresh water, and a large container under the group head and steam wand. Take the water filter out of the tank first. The solution shouldn't pass through it.
Entering descale mode
Most Breville machines have a dedicated descale mode rather than leaving you to run solution through by hand. On the Barista Express you typically enter it by holding a button combination at power-on. The Barista Pro shows a descale alert and a menu item on its LCD. The Bambino enters its cycle with a press-and-hold combination.
Once in descale mode, the machine walks the solution through the boiler, group head, steam wand and hot-water path in stages. Follow the prompts and empty the drip tray as it fills. It can overflow mid-cycle if you don't.
Rinsing properly
After the solution pass, refill the tank with fresh water and let the machine run its rinse stages to the end. If your model doesn't automate the rinse, run at least one full tank through the group head, steam wand and hot-water outlet before you brew anything.
Put the water filter back in, pull one sacrificial shot to clear any residue, and you're done.
How often Breville machines need it
Breville keys descaling frequency to water hardness: roughly every 2 to 3 months on moderately hard water, more often on hard water. If your model has a descale alert, respect it when it appears. The ThermoJet heaters in newer models are compact, and compact heaters scale up fast.
Day to day, filtered water plus a quick purge of the steam wand after each use keeps both scale and milk residue under control between cycles.