What is the correct order, from least to greatest, for the numbers 0.09, 0.099, 0.9, 0.009, and 0.091?

The first step is to write the numbers in a column, aligning the decimal points:
0.09
0.099
0.9
0.009
0.091
Next, fill in zeroes to give each number the same number of decimal places. This does not alter the number values in decimals as it does in whole numbers.
Since the whole number is the same (0) for all of these, you now just have to compare the number of “thousandths” (the numbers to the right of the decimal point) contained in each number: 90, 99, 900, 9, and 91. If you put those numbers in order, and remove any zeros from the right ends of the numbers, you will have the correct answer.