Here are a few examples of dominant chord voicings. Make sure you can see all the intervals between the chord notes and the root.
Open image in new window or listen to the chords (mp3)
Open image in new window or listen to the chords (mp3)
Be careful to avoid open strings and non-chord notes ringing. As a general rule you should prevent the ringing from non-chord notes by damping them with a finger fretting a lower string.