Tranquillity meditation is designed to calm the mind and body by focusing single-pointedly upon an appropriate object. You could focus on the image of an object you find calming or your breath, though either can be used with equivalent results.
In terms of how your body should be whilst meditating it is best to sit with your legs folded in front of you, sitting on a cushion will elevate your back and will help you meditation though it is not necessary, your hands should be placed palms upwards, either on your knees or one on top of the other, with your thumbs raised and touching with each other to form a circle or with your fingers if your hands are separate. Your back should be held straight but your shoulders should be relaxed and your head slightly inclined forward. Your eyes should be partially open and facing four feet in front of you. Lastly when you sit down find a position you can breathe naturally in, some positions restrict breathing, and feel comfortable in.
Now on to the meditation state itself; either envision an object and count your breath cycles to ten or higher depending on how well you can visualise objects or in fact concentrate on the breathing cycle itself, the part of the top of your mouth where you feel the breath is the best part to concentrate upon and again count the cycles for as long as possible or until you lose your focus. This meditation is designed to increase your focus and the length at which you can meditate so even though it’s simple in relation to meditations requiring you to clear your mind or go to extreme time periods it is a good stepping stone and will serve as a useful practice tool throughout your time as a Jedi.