Keep it simple or you’ll end up with a coffered ceiling and space below it, or no real space at all – three-piece buildups are usually both necessary and sufficient
 

At 8 linear feet of ceiling height, the coffers should be flattish to the ceiling and not built down more than a couple of inches (this home designing approach is less often applied and can be amazingly distinctive)
 

By 10 linear feet of ceiling height, the coffers can have depth (but needn't necessarily) of up to a foot or so
 

Be as symmetrical as you can, but you needn’t get obsessive, paying more attention to symmetricity at opposing perimeters (Autocad works swell at moments of coffered ceiling design)
 

If there are ceiling mounts, pendants, and recessed luminaires or ceiling fan(s) with which to contend, get ‘em in the centers of coffers as close as you can
 

Close perimeter gaps to walls and joints to ceiling with whatever’ll look good – sometimes caulk, sometimes simple moulding, especially if finished light-colored in stain or, more especially, in paint
 

Remember that dark stain can cover-up even moderately botched joining, often beyond casual observation – not so with lighter finishes, the toughest with which to contend is white or nearly white wherewith shadow lines MUST be closed
 

If you use wood rather than MDF, remember to twice-spray the raw wood lightly with shellac at any surface area that looks like it could be a bleeder when finish coats are in paint
 

If you use MDF rather than wood, remember to take extra care to smooth the volcanoes with patience and a light abrasion [Volcanoes here means MDF material displaced around the driven nails, puckering out all around the penetration like . . . well, like a tiny volcano.  They're hardly noticeable when the MDF surface is raw; however, once painted, the puckery volcanoes look like shoddy workmanship.  The mention of a light abrasion is to alert that MDF does not take well to heavy pressured sanding abrasion and not at all well to coarser grits . . . the abraded surface gets rough and ragged and is nearly impossible to restore to a smooth finish.]
 

You may mix wood and MDF when finishing in paint
 

Be discriminating in arranging the coffers, noting that a coffered ceiling defines space below it as special, on its own, noteworthy, different
 

Use the coffered ceiling to highlight aspects of functionality to the space below it, e.g., patterning differently over a dining table patterning to regulate downlight spacing, etc.
 

A coffered ceiling (and on down abutting walls) can distinctively transform a passage no matter the lengthwise extent of the coffering - even briefly applied at a major entrance or at a stair's first steps can make for a WOW moment that satisfies again and again
 

Grounds [the flat spaces between the coffers] needn’t be wooded-over as with strip trim or high-grade ply – precedent ceiling of gypsum board will do nicely – if you’re finishing in paint
 

Coffered ceiling in shallow profile may be more decorative in layout than a coffered ceiling with depth of profile – the notion being that with depth of profile, there should be a suggestion of apparent structure being trimmed-over . . . a consideration which should carry through sizing and spacing of coffered runs ranging with apparent strength of members covered-up