Write the number you want to factor at the top of the paper. In this example, we'll use 210.
Make the second level of the factor tree. Pick any number that divides evenly into the top number. You can choose any number; for example, if you divide 210 by 2, you get 105. So the second level of the factor tree consists of two branches, 2 and 105. Draw two lines branching out from beneath 210 in a teepee shape; write 2 under the line on the left and 105 under the line on the right.
Continue to the next level. The number 2 can't be broken down any further, so leave it alone and concentrate on 105. Since you can divide 105 evenly by 5 --- 5 times 21 equals 105 --- use that number to create the next level. Draw two lines branching out from 105 and write 5 under the left line and 21 under the right line.
Complete the fourth level of the factor tree, dealing with 5 and 21. Leave 5 alone, because it can't be broken down any further, and concentrate on 21. The latter is a product of 3 and 7, so draw two lines branching out from 21 and write 3 under the left line and 7 under the right line. Since neither 3 nor 7 can be broken down further, this is the final level of the factor tree.
Write down all of the numbers that could not be broken down further --- these are the factors of the top number. So, in this example, 2, 5, 3 and 7 are factors of 210.