You can use ratios to scale drawings up or down (by multiplying or dividing).
Example from the image above:
To draw a horse at 1/10th normal (smaller) size, multiply all sizes by 1/10th or divide the sizes by 10. This horse in real life is 1500mm high and 2000 mm long, so the ratio of its height to length is 1500 : 2000
So what is that ratio when you draw it at 1/10th normal size (smaller) size?
Answer:
1500 : 2000 = 1500×1/10 : 2000×1/10 = 150 : 200
OR
1500 :10 : 2000 :10 = 150 : 200
You can make any reduction/enlargement you want that way.
You will sketch using grid line and also applying your skills and understanding of scaling. Following this instruction
1. See the image above in ratio of 6:4 , you can enlarge or reduce the image by multiplying or dividing the size to make bigger or smaller scale. or ask teachers for other image that you prefer.
2. Draw the new grid line using new size scale in your sketchbook
3. Start to sketch with the help of gridline. see this link to learn more about how to draw using grid http://www.art-is-fun.com/grid-method.html
4. Showing off your sketch to your friends
Have Fun!