Review the basics of fractions. Ideally, if students are ready to learn how to multiply fractions, they should understand the basic concept that fractions are ratios, but review can't hurt. Be sure to identify the numerator (the top number) and denominator (the bottom number) and, importantly, make sure to explain that when you multiply fractions that the product will be smaller than either of the fractions that were being multiplied.
Start with a simple example, ideally one that does not require any simplification. For example, you can demonstrate how to multiply "2/3" by "1/3" by simply showing students that they multiply the numerators together and then multiply the denominators in order to get "2/9" as an answer.
Use an example that requires simplification. The next level of complexity for multiplying fractions is needing to simplify answers, which is when the numerator can be divided into the denominator; because fractions are ratios, the values are still the same. For example, multiply "2/3" by "1/4" to get "2/12," and then simplify it to "1/6."
Show how to cross-simplify. Cross-simplifying is a trick that can be used to help make multiplying fraction easier, though it is not essential. You cross-simplify the numerator of one fraction with the denominator of the other one being multiplied; you do this by dividing by the largest term that can be evenly divided into both numbers. For example, if you are multiplying "2/3" by "3/4," you can make the 3 in both numbers into 1 by dividing by 3 by 3, and you can divide the 2 by 4 to get 1/2.; 1/2 times 1 is 1/2.