‘Just sitting’ is the literal translation of the Japanese word shikantaza, which is the term for the central practice in Sōtō Zen Buddhism. Zazen is another word that is often also heard, meaning ‘seated meditation’ and it includes shikantaza as well as other practices.
As humans, we habitually compare our experience to our expectations of reality, how we think life should be at any moment. The Buddha pointed to this desire for life to be other than it is as the basis of human suffering. Shikantaza is an opportunity to just sit with life as it is directly and become intimate with it rather than be grasping at the good parts and pushing the bad parts away. We therefore experience life in a state of total fullness, however we are feeling on a particular day. Whether we are strong, weak, in pain or not in pain, there is nothing lacking in any moment, everything is complete and whole just as it is. We are, in essence, the whole universe sitting itself (yes, I know, that sounds incredibly Zenny and pretentious but it does capture the essence of shikataza for me, so can we go with it until I think of something better?). Zen teacher Taigen Dan Leighton refers to shikantaza as an enactment ritual of Buddha sitting which is explained more in his essay given below.
In much of life we are often very goal-oriented but in shikantaza we drop ideas of striving or anything to achieve, instead just resting in what is happening right now at this very moment. There is nothing to achieve and nowhere to go. You have nothing to do except be with life unfolding, moment by moment.It is not that we do not make any effort and zone out, but neither do we push to achieve anything. Instead, it is good to retain a state of relaxed awareness and the idea of the universe sitting itself in that the effort naturally rises through us rather than being something we have to add.
When thoughts arise, as they inevitably will, that is totally fine and we neither cling to them nor chase them but let them be just as they are.
If you feel tired or agitated when sitting, you can just be aware of that. If you feel really sleepy it may be better to stop and just let your body sleep. Keeping a set time for your practice is a good idea as it gets you into a habit. Many people find that first thing in the morning is good. Personally, my body takes time to get going. Others like early evening. Too late at night often leads to sleepiness.
A good time to sit is for 20-30 minutes but it is fine to sit for shorter periods and build up if and when you can. If ten minutes sitting a day is what works for you, that is fine.
Have a shrine or not, depending on your feelings about it. It can be as simple as a picture of the Buddha, a vase of flowers, or even a rock. Use incense and candles if you wish. The Insight Timer app is a good way to both time your meditation and connect to fellow sitters.

For those of us with physical health or disability issues, just ‘sitting’ may not be possible, or at not in the conventional postures. I outline some alternatives here. The important thing is to be comfortable. In the suggested guides to practice below, sitting will be taken as the norm but do not worry if you cannot physically sit. That does not mean you cannot practice shikantaza, it just becomes ‘just laying’ or ‘just propped up with pillows’! Sitting is also the usual phrase used for shikantaza practice (and other forms of meditation) but just translate it as it applies to you.