lucy24 - 8:56 pm on Nov 2, 2011 (gmt 0)
You've got at least two completely unrelated questions, so it may be best to ask them separately.
#1 guidelines for designing a mobile-friendly version of the site
#2 how and when to redirect
The second question is easier. Or, at least, can be answered in fewer words. It's done in your .htaccess via device detection. Look for keywords in the user-agent string; you can pick up an amazing lot just by the case-sensitive sequence "iP". Others are "mobile" and "mini".
It isn't the single most common question in the Apache forum, but there are enough of them that a Forums search will turn up the recurring themes. One part is: make sure the user has the option of not being redirected if they explicitly ask for the non-mobile version.
But first, design your mobile-friendly site.