If you can see through the language, repetitiveness and overlook some of the context setting, the best resource is here.

http://www.greatwesternvehicle.org/ati_website/canon/sutta/m...

It is supposedly a transcript from the buddha discources.

There is nothing "spiritual" about it and is very systematic (basically lists of what to do) and worded very carefully. Most practaces I have gone to seem to lack the precice details.

Here is the main part, sorry for the length, but I tried to summarise and it would not do.

"Now how is mindfulness of in-&-out breathing developed & pursued so as to be of great fruit, of great benefit?

"There is the case where a monk, having gone to the wilderness, to the shade of a tree, or to an empty building, sits down folding his legs crosswise, holding his body erect, and setting mindfulness to the fore.[1] Always mindful, he breathes in; mindful he breathes out. "

"[1] Breathing in long, he discerns that he is breathing in long; or breathing out long, he discerns that he is breathing out long. [2] Or breathing in short, he discerns that he is breathing in short; or breathing out short, he discerns that he is breathing out short. [3] He trains himself to breathe in sensitive to the entire body,[2] and to breathe out sensitive to the entire body. [4] He trains himself to breathe in calming bodily fabrication (the breath),[3] and to breathe out calming bodily fabrication.

"[5] He trains himself to breathe in sensitive to rapture, and to breathe out sensitive to rapture. [6] He trains himself to breathe in sensitive to pleasure, and to breathe out sensitive to pleasure. [7] He trains himself to breathe in sensitive to mental fabrication (feeling & perception), and to breathe out sensitive to mental fabrication. [8] He trains himself to breathe in calming mental fabrication,[4] and to breathe out calming mental fabrication.

"[9] He trains himself to breathe in sensitive to the mind, and to breathe out sensitive to the mind. [10] He trains himself to breathe in satisfying the mind, and to breathe out satisfying the mind. [11] He trains himself to breathe in steadying the mind, and to breathe out steadying the mind. [12] He trains himself to breathe in releasing the mind, and to breathe out releasing the mind.[5]

"[13] He trains himself to breathe in focusing on inconstancy, and to breathe out focusing on inconstancy. [14] He trains himself to breathe in focusing on dispassion [literally, fading], and to breathe out focusing on dispassion. [15] He trains himself to breathe in focusing on cessation, and to breathe out focusing on cessation. [16] He trains himself to breathe in focusing on relinquishment, and to breathe out focusing on relinquishment.