What's the best way to approach science faculty about doing research in their lab (undergraduate) ?

be kind, oh yea bring a fat juicy rat

You simply go to the professor during office hours and inquire that you're interested in doing undergrad research the following semester.  It's best to see a prof you are or had took a course from and did well in, as that provides some positive light that you're capable of the task.  I would not use email since the prof often cannot make the name/face connection and it's too easy for them to just ignore you.  Nonetheless, be ready for rejection, as not all profs are interested in undergrad research supervision nor are they obligated to do so.  Some profs also aren't really doing any active research at all so have no need for anyone.   Also, in some schools, departments, or individual professors,  there may be a competitive process for undergrad research involving a formal application process since there are so many students trying to do undergrad research.