When changing between the C and D7 chords the first finger does not move. The note played by the first finger (a C note) is common to both chords. The second and third fingers move to their new position and the first finger acts as a pivot. Use the slide finger when changing between G and D7.
Always end a chord progression by strumming the first chord.
The following chord progression contains the three chords you have learned so far. There are four quarter note strums in each bar as indicated by the rhythm pattern.
G7
To play the G7 chord, place the first, second and third fingers of your left hand as shown in the diagram. Strum all six strings.
This symbol is called common time. It means exactly the same as *.