G/2 is a half-beat G, and G2 is a double-beat G. Experiment with beat lengths. For example, try three G/3 instead of two G/2.
In some songs, you will notice notes like G/ without a number. A / without a number is an abbreviation for 1/2 and a // without a number is an abbreviation for 1/4. Experiment with beat notation.
B and C' complete the phrase. Try finishing the phrase, getting the rhythm right. Remember you can use Z for a rest.