A flashcard has one job: to make you retrieve one thing from memory. Every rule below is a way of making that retrieval clean, quick and unambiguous.
1. One thing per card
This is the most important rule, often called the minimum information principle. A card that asks for a word, its three meanings and an example sentence is really five cards. When you get part of it right and part wrong, you cannot answer honestly, and the review schedule goes wrong.
Split it. One word and one meaning. A second card for a second meaning if it matters.
2. Write the meaning in your own words
Copying a dictionary definition feels thorough, but it gives memory nothing to hold. Rewriting the meaning in your own plain language forces you to understand it, and the result is shorter and easier to recall.
"Fortuitous - happening by chance, not necessarily lucky" is better than a full dictionary entry.
3. Add the sentence you found it in
A word on its own is abstract. The sentence where you met it - from a book, a meeting, an article - carries context and a little memory of the moment. That context is one more path back to the word.
If you did not find it in a sentence, write one about your own life.
4. Keep the prompt short
The front of the card should be recallable from a glance. If you have to read a paragraph before you can try to answer, most of your effort goes into reading, not remembering.
5. Avoid lists
"Name the seven…" cards are notoriously hard to learn, because remembering six out of seven feels like both success and failure. If a list matters, break it into cards that each ask for one item in a distinct context.
6. Use gaps for phrases
For expressions, a sentence with a gap works well: "We did a ____ calculation before the meeting." This is called a cloze card. It trains you to produce the phrase in the kind of sentence where you will actually use it, rather than just recognise it.
7. Make it unambiguous
If a prompt has two reasonable answers, you will argue with yourself at every review. Add just enough context to rule out the other answer - a hint about the situation, or the first letter.
8. Write fewer, better cards
Adding cards is satisfying. Reviewing a pile of badly written ones is not, and that pile is where most flashcard habits die. It is far better to write one good card for something you genuinely want to keep than ten quick ones for things you do not.
Where your own cards fit
The most valuable cards are usually the ones you write yourself: the phrase a colleague used, the word you had to look up in a book. They come with your own context attached. Write them carefully, let them join your regular reviews, and they tend to stay longer than anything you could download.