im not an anapanasati-wallah, but there are definitely at least a few on this board - triplethink and nathan28 (both named nathan actually) spring to my mind. i'd say send a pm to them.
otherwise, here's my best shot, adapted from bhante vimalaramsi's style:
sit in a comfortable position (floor or chair, doesnt matter) that is also erect but not stiff. find a kind of inner balance that makes it easy to sit up without much effort. then gently and patiently turn attention to the breathing process. watch how the breathing affects the whole body, breathing in and out. breathe in, relax. breathe out, relax. dont worry about the condition of the breath (whether its deep or shallow, slow or fast, smooth or ragged, etc). just keep following along, staying attentive, staying mindful. breathe in and feel the body relax. breathe out and feel the body relax. keep that relaxation going along with the awareness of the breath and kind of sink in to it. remember that this is a trial and error process and the mind will wander, get distracted, etc. no problem. just keep coming back. getting too involved in thinking about how well you're doing or not doing = not so interesting. sinking into whatever relaxation you can feel at this very moment = much cooler. try to stay focused, but dont make thoughts or thinking the enemy.. no enemies here buddy, just awareness of the breath and the relaxing happiness that attending to it provides.
keep doing this for awhile, either for a set period of time or just til whenever you want to stop. and remember these are loose instructions.. there are so many ways of doing it, and so many different ways of doing it well. anything that feels intuitively correct probably is, so if the above doesnt resonate completely with you, tweak it.
ps dont forget to find enjoyment in it and have some fun

pps if anyone sees some common sense advice i forgot, pls feel free to add it