Simran is remembrance, the repetition of the name, and it is the central practice the Gurus prescribe. Everything else in Sikh practice is arranged around it.
It can be silent, whispered, or sung in company. Kirtan is simran done collectively with music. The amrit vela, the hours before dawn, is the prescribed time.
The Guru Granth Sahib Ji is emphatic that it is not a technique that produces a result mechanically and is not a substitute for conduct. The critique of empty repetition runs through the volume as hard as the critique of ritual bathing.
It requires no place, no equipment, no intermediary and no fee, which is doctrinally the point and also a practical fact about a religion that was founded among working people and spent a century being hunted.
The whole apparatus of gurdwaras, committees, calendars and disputes exists around a practice that needs none of it.
The practice is repetition, of the Name or of a shabad, aloud or under the breath or silently, and the rehat sets it in the amrit vela without confining it there.
It is done alone and it is done together. The sangat's collective simran, one word or line repeated by a room for an hour, is a different thing from the individual practice and both carry the same name.
The tradition is insistent that it is not a technique that works mechanically. What is repeated is a relationship, and the texts return constantly to the point that repetition without attention is nothing.