Mixed Chords or Slash Chords

This is a 2.4 minute video response from Matt Brown on Nov 1st, 2008

Question:

"Djhectichector says, "I'm currently practicing on backing tracks I've bought and i have noticed on the corresponding chord sheet there are chords such as D/F# etc. How do i work out the notes in that chord? Sorry if this is a bit OTT for my level (Half way through modes) Typical song containing this chord by the way is (Wonderful tonight by Eric Clapton.) I tend to use a D major but it don't sound right. The solo I've got to the tea but D/F# Messes me up. Please Help!""