In Chapter 6 ("What to Do with Your Body") of Mindfulness in Plain English the author, Bhante Henepola Gunaratana, offers nuts-and-bolts advice on how to sit — with the honest disclaimer that all the mechanics are merely a tool, not a goal:

The purpose of the various postures is threefold. First, they provide a stable feeling in the body. This allows you to remove your attention from such issues as balance and muscular fatigue, so that you can center your concentration on the formal object of meditation. Second, they promote physical immobility which is then reflected by an immobility of mind. This creates a deeply settled and tranquil concentration. Third, they give you the ability to sit for a long period of time without yielding to the meditator's three main enemies—pain, muscular tension and falling asleep.

The most essential thing is to sit with your back straight. ...

And, whether you sit on a chair or in full lotus:

... remember your objectives. You want to achieve a state of complete physical stillness, yet you don't want to fall asleep. Recall the analogy of the muddy water. You want to promote a totally settled state of the body, which will engender a corresponding mental settling. There must also be a state of physical alertness, which can induce the kind of mental clarity you seek. So experiment. Your body is a tool for creating desired mental states. Use it judiciously.

(cf. Posture (2009-06-05), Afraid of Chairs (2010-10-15), With Dignity (2010-12-04), Just Sitting (2011-05-21), Wait for the Breath (2013-07-09), Eleven Rules for Mindfulness (2015-11-07), Look at Each Second (2015-11-17), Mindfulness, Concentration, and Distraction (2015-11-23), No Me (2016-01-18), ...) - ^z - 2016-03-23