Mindfulness of Breathing, called ānā-pāna is described on p. 285 of the Path of Purification (Visuddhi-Magga), translated by Bhikkhu Nanamoli. It runs as follows:
It has been described by the Blessed One as having sixteen bases thus: "And how developed, Bhikkhus, how practised much, is concentration through mindfulness of breathing, both peaceful and sublime, an unadulterated blissful abiding, banishing at once and stilling evil unprofitable thought as soon as they arise?
Here, Bhikkhus (monk), a Bhikkhu, gone to the forest or to the root of a tree or to an empty place, sits down; having folded his legs crosswise, set his body erect, established mindfulness in front of him, ever mindful he breathes in, mindful he breathes out.
(I) Breathing in long, he knows "I breathe in long"; or breathing out long, he knows "I breathe out long".
(II) Breathing in short, he knows "I breathe in short "; or breathing out short, he knows "I breathe out short".
(III) He trains thus "I shall breathe in experiencing the whole body"; he trains thus "I shall breathe out experiencing the whole body".
(IV) He trains thus "I shall breathe in tranquillising the bodily formation"; he trains thus "I shall breathe out tranquillising the bodily formation".
(V) He trains thus "I shall breathe in experiencing happiness"; he trains thus "I shall breathe out experiencing happiness".
(VI) He trains thus "I shall breathe in experiencing bliss"; he trains thus "I shall breathe out experiencing bliss".
(VII) He trains thus "T shall breathe in experiencing the mental formation"; he trains thus "I shall breathe out experiencing the mental formation".
(VIII) He trains thus "I shall breathe in tranquillising the mental formation"; he trains thus "I shall breathe out tranquillising the mental formation".
(IX) He trains thus "I shall breathe in experiencing the (manner or) consciousness", he trains thus "I shall breathe out experiencing the (manner of) consciousness".
(X) He trains thus "I shall breathe in gladdening the (manner of) consciousness", he trains thus "I shall breathe out gladdening the (manner of) consciousness".
(XI) He trains thus "I shall breathe in concentrating the (manner of) consciousness"; he trains thus "I shall breathe out concentrating the (manner of) consciousness".
(XlI) He trains thus "I shall breathe in liberating the (manner of) consciousness"; he trains thus "I shall breathe out liberating the (manner of) consciousness".
(XIII) He trains thus "I shall breathe in contemplating impermanence"; he trains thus "I shall breathe out contemplating impermanence".
(XIV) He trains thus "I shall breathe in contemplating fading away"; he trains thus "I shall breathe out contemplating fading away".
(XV) He trains thus "I shall breathe in contemplating cessation"; he trains thus "I shall breathe out contemplating cessation.
(XVI) He trains thus "I shall breathe in contemplating relinquishment"; he trains thus "I shall breathe out contemplating relinquishment". (S.V.321-2)".
This mindfulness, unlike other kammathāna subjects, is for both Samatha and Vipassanā, and goes from Samatha to Vipassanā. However, it is too very difficult, as the language is very involved.
The best thing is to follow the Jhāna arisings as described in Chapter XIV (p. 91) and to follow the Vipassanā Meditation as described in Chapter VII (p. 59).
Regarding the subject of Concentration, we should be acquainted with the general terminology. It will help you to understand this book and other books.
The attempt made by the aspirant in fixing his mind on the object is called Parikamma-Bhāvanā. There are many kinds of objects and it is the first concentration on the object that is called Parikamma Bhāvanā. When the object, called the symbol, is thoroughly grasped by the Mind, and appears as if the aspirant sees it with the eyes open, he is said to have obtained the visualised image; it is a concept called Uggaha-Nimitta, which is the mental replica of the symbol.
The attempts have to make the visualised object clearer and brighter than the actual object as seen by the open eyes is called Upacāra-Bhāvanā. This brighter concept is called the Pathibhaga-Nimitta, which is the "after-image".
As soon as the 5 Hindrances are suppressed, Upacāra-Samādhi is obtained and is otherwise called Access or Proximate Samādhi. The Mind is not yet steady even at this stage. Upacāra-Samādhi is proximate to Appanā-Samādhi.
Appanā-Samādhi is full fledged concentration; the aspirant can stay in this stage for as long as he wants, for the 5-Hindrances have been eliminated. With the attainment of Appanā-Samādhi, he has attained the 1st Jhāna.
Parikammā - Upacāra - Anuloma - Gotrabhū - Appanā.
The Mind-Consciousness now takes the Patibhaga-Nimitta and prepares it for the first stage of Jhāna.
In turn we get Upacāra (Access) and Anuloma, which is called Adaptation consciousness. Then comes Gotrabhū, where the thought-moment transcends the sensuous plane. The former lineage has been cut off and the Mind evolves the lineage of the Form Plane. This is immediately followed by Appanā-Samādhi, which is Ecstatic Concentration. This is First Jhāna, otherwise called Rūpa-Jhāna.