Place a lid or plate that fits into the container on top of the surface. Add a weight like a sterilized rock, a jar or glass bottle filled with water. OR, if using a small batch fermentation kit, add the weight(s) to the jar.
There should be enough brine to just cover the contents when weighed down. It is normal for more brine to form in the first day or so after putting up.
Cover container with a dish towel or tea towel to keep out flies and dust. Secure with a rubber band, twist ties or elastic strap. Or if using an airlock lid, add it to the top of the container.
Label your container with the contents and date started. (I use blue painter’s tape and a permanent marker, and I always stick the label on the side of the container, rather than the lid. (Those lids have a notorious habit of switching jars when you’re not looking, usually in the middle of the night.)
Stash it in a cool place, out of direct sunlight.
Sauerkraut can ferment in as little as 5 days. Typically, I ferment sauerkraut two weeks in warm weather, or four weeks in cooler weather. Fermentation speed varies with the seasons and the climate.