John

If you're experienced at playing in open G you already know most of the chord shapes you need for open C. Just move your open G chord shapes one string to the treble side of the guitar. You don't get the same chord in name, but you get the same chordal relationship (relative minor, dominant etc.).

For instance, take whatever shape you use for an Em in open G. In open C move that same shape one string to the treble side and you have an Am. Do the same with whatever shape you use for a C chord in open G and you have a Bb in open C.

Find ways to add in sone bass notes on that low C string and you're home free.

Bruce