Hi Peter,
The slash symbol / means that the chord is simply a combination of two standard chords. I often play the C/D chord, for example. The first note indicated is the bass note, in this case C.
I play this by making an open D chord, but putting a finger on the C (third fret of the A string) instead of the open D, which is muted by the finger on the C note.
If this seems too difficult, don't worry about the F# of the D chord (second fret of the high E string.) But this simple chord sounds very cool, especially when played in a chord progression after the D or G chords.
For a C/B however, I would probably play a barre chord-style C, only with a finger on the B bass note. This kind of chord would probably sound best when followed by a C barre chord.
The G/F# is another chord where I would use the second note indicated as the bass note. Play a simple open G chord, only put a finger on the bass F# note (second fret low E string.) If you play open G chords with a finger on the bass B note, you can drop that and mute it with the finger you have on the F#.
Hope this helps!
Karl