Draw six vertical lines to represent your strings. Also, draw three horizontal lines, perpendicular to the vertical lines. You'll repeat this step for each of the chords you want to indicate.
Indicate the C chord by drawing a circle with a 1 inside it on the first fret on the string second from the right. Draw a circle with a 2 inside it on the second fret on the string third from the left. Draw a circle with a 3 inside it on the third fret on the string second from the left. Above the fretboard, draw an X above the first string on the left, and O's above the first string on the right and third string from the right. You've now indicated the C chord.
Indicate the D chord by drawing a circle with a 2 inside it on the second fret on the string farthest to the right. Draw a circle with a 3 inside it on the third fret on the string one string away from the string farthest to the right. On the string to the left of that, on the second fret, draw a circle with a 1 inside of it. Above the fretboard, draw X's above the string farthest to the left and the string right next to that. Draw an O above the string two strings from the left.
Indicate the G chord by drawing a circle with a 4 inside it on the third fret on the string farthest to the right. Draw a circle with a 3 inside it on the third fret on the string farthest to the left. On the string to the right of that, draw a circle with a 2 inside it on the second fret. Draw O's above the fretboard on the strings that don't currently have circles with numbers on them.