Mentally you recite 'bud' on the inhalation and 'dho' for the exhalation. Buddho means "Awakened One". The purpose of using the word "Buddho" along with the breath is a technique that helps the mind focus easily on one-pointedness. First of all, the practitioners of this method should practise samatha to calm the mind. Then they begin to practise vipassana by using the calmed mind to concentrate on the reality of all things truly as they are. One important element of this technique that practitioners should do before trying out meditating is to investigate the body as the compound of the four elements and the Five Aggregates. Observe your thoughts regarding the body as a desirable thing to cling to, as impermanence and non-self. Contemplate the body like this until the mind accepts these realities. It is easier if the practitioner attains a concentration first and then investigates the body.
Luang Phu Mun's Samatha - Vipassana Meditation
This is our foundation: to have sati, recollection, and sampajañña, self-awareness, whether standing, walking, sitting, or reclining. Whatever arises, just leave it be, don't cling to it. Be it like or dislike, happiness or suffering, doubt or certainty... Don't try to label everything, just know it. See that all the things that arise in the mind are simply sensations. They are transient. They arise, exist and cease. That's all there is to them, they have no self or being, they are neither ''us'' nor ''them.'' They are not worthy of clinging to, any of them. - Ajahn Chah