Why do I have my German students make flashcards? It may seem like an "old-school", but there are a lot of really, well-researched reasons why making flashcards can help students learn vocabulary. There are also many little tricks--activities & review games--that one can use with their cards after they've been made. For studying German, I always recommend that student either color code their words, or put their vocabulary on colored cards.
For example, German nouns are divided into 3 genders--masculine, feminine, and neuter. I recommend using blue ink (or marker) for the masculine words, red or pink for feminine, and green (or brown or black) for neuter words. Because the genders of nouns become a very important topic beyond level 1, I stress from DAY 1 that students use these methods in order to get in the habit of learning the "der, die, and das" with the noun. [Ex. der Computer, die Fahne, das Buch] Another tip is to look at the vocabulary list--words are always grouped and organized by gender or part of speech--like verbs, adjective/adverbs. I have had students remember where a word is located on the page & that triggered the memory of the meaning and of the gender. Why? For example--a word was masculine because the student remembered it was on the top left side of the page. This kind of spacial recognition is also very useful in learning new vocabulary.

Using Flashcards--Different Strategies

Univ. of Michigan--Vocabulary Learning Tips

TEACHING VOCABULARY USING FLASHCARDS AND WORD LISTS

Voxy Blog--Pros and Cons of Flashcards

Effectiveness of Quizlet