From the Visuddhimagga:
So, after seating himself in the way stated, he should review the dangers in sense desires in the way beginning, “Sense desires give little enjoyment” (M I91) and arouse longing for the escape from sense desires, for the renunciation that is the means to the surmounting of all suffering. He should next arouse joy of happiness by recollecting the special qualities of the Buddha, the Dhamma,and the Sangha; then awe by thinking, “Now, this is the way of renunciationentered upon by all Buddhas, Paccekabuddhas and noble disciples”; and then eagerness by thinking, “In this way I shall surely come to know the taste of the bliss of seclusion.” [125] After that he should open his eyes moderately, apprehend the sign, and so proceed to develop it. If he opens his eyes too wide, they get fatigued and the disk becomes too obvious, which prevents the sign becoming apparent to him. If he opens them too little, the disk is not obvious enough, and his mind becomes drowsy, which also prevents the sign becoming apparent to him. So he should develop it by apprehending the sign (nimitta), keeping his eyes open moderately, as if he were seeing the reflection of his face (mukha-nimitta) on the surface of a looking-glass.
29. The colour should not be reviewed. The characteristic should not be given
attention.But rather, while not ignoring the colour, attention should be given by setting the mind on the [name] concept as the most outstanding mental datum, relegating the colour to the position of a property of its physical support.
That [conceptual state] can be called by anyone he likes among the names for earth (pathavì) such as “earth” (pathavì), “the Great One” (mahì), “the Friendly One” (medinì), “ground” (bhúmi), “the Provider of Wealth” (vasudhá), “the Bearer of Wealth” (vasudhará), etc., whichever suits his manner of perception. Still “earth” is also a name that is obvious, so it can be developed with the obvious one by saying “earth, earth.” It should be adverted to now with eyes open, now with eyes shut. And he should go on developing it in this way a hundred times, a thousand times, and even more than that, until the learning sign arises. When, while he is developing it in this way, it comes into focus as he adverts with his eyes shut exactly as it does with his eyes open, then the learning sign is said to have been produced. After its production he should no longer sit in that place;10 he should return to his own quarters and go on developing it sitting there. But in order to avoid the delay of foot washing, a pair of single-soled sandals and a walking stick are desirable. Then if the new concentration vanishes through some unsuitable encounter, he can put his sandals on, take his walking stick, and go back to the place to re-apprehend the sign there. When he returns he should seat himself comfortably and develop it by reiterated reaction to it and by striking at it with thought and applied thought.
Fire Kasina:
Anyone who wants to develop the fire kasióa should apprehend the sign in fire. Herein, when someone with merit, having had previous practice, is apprehending the sign, it arises in him in any sort of fire, not made up, as he looks at the fiery combustion in a lamp’s flame or in a furnace or in a place for baking bowls or in a forest conflagration, as in the Elder Cittagutta’s case. The sign arose in that elder as he was looking at a lamp’s flame while he was in the Uposatha house on the day of preaching the Dhamma.
6. Anyone else should make one up. Here are the directions for making it. He should split up some damp heartwood, dry it, and break it up into short lengths.
He should go to a suitable tree root or to a shed and there make a pile in the way done for baking bowls, and have it lit. He should make a hole a span and four fingers wide in a rush mat or a piece of leather or a cloth, and after hanging it in front of the fire, he should sit down in the way already described. Instead of giving attention to the grass and sticks below or the smoke above, he should apprehend the sign in the dense combustion in the middle.
He should not review the colour as blue or yellow, etc., or give attention to its characteristic as heat, etc., but taking the colour as belonging to its physical support, and setting his mind on the [name] concept as the most outstanding mental datum, and using any among the names for fire (tejo) such as “the Bright One” (pávaka), “the Leaver of the Black Trail” (kaóhavattani), “the Knower of Creatures” (játaveda), “the Altar of Sacrifice” (hutásana), etc., he should develop [the kasina] by using [preferably] the obvious “fire, fire.” As he develops it in this way the two signs eventually arise in him as already described. Herein, the learning sign appears like [the fire to keep] sinking down as the flame keeps detaching itself. [172] But when someone apprehends it in a kasina that is not made up, any fault in the kasina is evident [in the learning sign], and any firebrand, or pile of embers or ashes, or smoke appears in it. The counterpart sign appears motionless like a piece of red cloth set in space, like a gold fan, like a gold column. With its appearance he reaches access jhána and the jhána tetrad and pentad in the way already described.