Purchase a set of 3x5 index cards. Basic white index cards work well, but if you want to add to the visual experience of this teaching tool, consider using colored index cards.
Use a marker to print information on one side of the index cards. For example, if you are using the flashcards to teach vocabulary words, write the words on one side of the index cards. If you are using flashcards to teach multiplication, write multiplication problems on one side of the cards.
Flip the cards over. Use a marker to write the answer to the information written on the opposite side of the index card. For instance, for vocabulary flashcards, write the definition of the vocabulary words. For multiplication problems, write the answer to the problems.
Use the cards as-is or laminate them. If the cards are only going to be used once or by one person, they likely will be fine if left as-is. If the cards are going to be used by a classroom, or if they will be used throughout the school year, laminate them to make them more durable.