I thought I would share with you the "Brief Rule of St. Romuald", the founder of the Camaldolese Order of hermits who practice a strict form of eremetic asceticism. The Rule is probably the shortest monastic rule of all the many religious orders of the Western Church, but it is truly the deepest and one that might resonant with one who sits Shikantanza.
"Sit in your cell as in paradise, put the whole world behind you and forget it;
like a skilled fisherman on the lookout for a catch keep a careful eye on your thoughts.
The path you follow is in the psalms - do not leave it, if you have come with a novice's
enthusiasm and cannot accomplish what you want, take every chance you can find to sing
the psalms in your heart and to understand them with your head; if your mind wanders
as you read do not give up but hurry back and try again.
Above all realize that you are in God's presence; hold your heart there in wonder as if
before your sovereign.
Empty yourself completely; sit waiting, content with God's gift,
like a little chick testing and eating nothing but what its mother brings."
It is to things like this that I am able to point, written in the 11th century, when I am asked how I can practice Zen.