Put your palms together, with fingertips at eye level. Hands are one hand-width from your nose. Elbows are held out slightly from the body.
Fold your left thumb into your left palm and curl your fingers over it to make a fist. Cover your left hand with your right and hold your hands in front of you against the body, with forearms parallel to the floor.
Holding a chant book
It's always fine to use a book for chanting, even if others know the text by heart. There's no need to hide or minimize your book, or to try somehow to balance it on your knee or zabuton to put your hands in gassho while chanting.
Sit up straight and hold your book upright in front of you, with the three middle fingers on the outside and thumb and little finger on the inside. Keep your book on your zafu or otherwise avoid putting it on the floor when you're not using it.
If you’re not using a book during chanting, put your hands in gassho or the cosmic mudra (hokkaijoin) as appropriate.
Fold your left thumb into your left palm and curl your fingers over it to make a fist. Cover your left hand with your right and hold your hands in front of you against the body, with forearms parallel to the floor. Turn your hands so that the back of your hand is up and the palms are down. We put our hands in isshu during kinhin (walking practice in the zendo).
There's a lot of bowing in Zen practice. Bowing is an opportunity to practice respect, humility and selflessness. Sometimes we bow to the Buddha or bodhisattvas, sometimes to a teacher, and sometimes to each other. We even bow to our food before we serve and eat it.
Bowing is not a worshipful activity that invites us to become obsequious, pompous or self-conscious. We don't let our egos get in the way by comparing our bows with others'. Neither do we bow carelessly, tossing off a bow on the way to something or somewhere else. When it's time to bow, we just bow simply, sincerely and wholeheartedly..