Put the berries and salt in a vacuum bag.

Make sure the bag is long enough to be reopened and resealed.

Shake it well together.

Distribute the berries so they lie in a single layer as much as possible.

Seal the vacuum bag.

Let them sit in a warm place. Ideally, they should be at 28 degrees Celsius (82 degrees Fahrenheit) for 5-6 days.

If you don't have that temperature option, let them ferment at room temperature (about 20-22 degrees Celsius (68-72 degrees Fahrenheit)) - it will take a few days longer.

During the process, the bag will swell and need to be "burped," i.e., the air needs to be let out. This is a bit more complicated than if you had a canning jar where you just open the lid. Here you'll need to cut a small hole in the top of the vacuum bag to let the air out. Then it needs to be resealed.

Taste them; when they are a bit salty and quite tart, they should be taken out.

Strain them in a fine-mesh strainer and now they should be stored in the fridge.

They can keep for a few days. Alternatively, they can be vacuum-packed again and frozen.