Most flashcard decks fail because of the cards, not the app. A card that asks for too much, or asks something vague, is hard to answer, hard to grade honestly and annoying to review. A card that asks one clear thing is quick and gives the scheduler clean information. The difference between the two is mostly craft, and the craft is learnable.

In short

Make each card ask for one small thing. This is the minimum-information principle.
Ask a question with a specific answer. If you can't tell whether you were right, the card is too fuzzy.
Break lists and sets into separate cards instead of one big one.
Add context, an example or an image when a bare fact would be arbitrary.
Write cards in your own words after you understand the material, not before.
Review and fix cards you keep missing: the problem is usually the card.

The minimum-information principle

The best-known rule comes from Piotr Wozniak's article on formulating knowledge, which lists twenty rules for writing material for spaced repetition (Twenty rules of formulating knowledge). One of them is to stick to the minimum information principle: formulate material as simply as possible so that each repetition requires retrieving only a small amount of information. The goal is for a single review to ask for the minimum from memory.

Why does this matter? Simple items are easier to remember because your memory handles them the same way each time, and they are easier to schedule because each can be reviewed at its own pace. A big card that mixes five facts can only be answered "right" or "wrong" as a whole. If you get four of the five, the scheduler sees a miss, and the whole card comes back more often than it needs to.

Example: splitting a card

A weak card:

Front: Describe the water cycle.
Back: Evaporation from oceans and lakes, condensation into clouds, precipitation as rain or snow, collection in rivers and groundwater, and the cycle repeats.

A better set, each its own card:

Front: What is the name for liquid water turning into vapour at the surface? Back: Evaporation.
Front: What happens to water vapour as it rises and cools? Back: It condenses into clouds.
Front: What are the common forms of precipitation? Back: Rain, snow, sleet, hail.
Front: Where does precipitation collect? Back: Rivers, lakes, oceans, groundwater.

Each is quick. A miss on one points at exactly what you do not know.

Write questions with clear answers

A good test of a card: could two people disagree about whether an answer is correct? If so, the card is too vague.

Vague: "What do you know about the French Revolution?"
Clear: "In what year did the French Revolution begin?"

Specific cards also protect against self-deception. It is easy to look at a long answer and think "yes, roughly that". With a short, specific answer, you either had it or you did not.

Handle lists and sets with care

Wozniak's rules specifically say to avoid sets and enumerations. A card that asks you to recite every item of a group is one of the hardest to answer and grade, because it is easy to remember three of seven and still feel you knew it.

Options that work better:

One item per card, each with its own question: "Which cranial nerve controls eye movement?"
Overlapping partial cards, where each card asks for one member given a cue.
A different kind of question, such as "What do these three have in common?"

Bulk paste helps here: a list of front and back pairs becomes many small cards in one pass instead of one giant card (bulk-importing flashcards).

Make facts less arbitrary

Isolated facts are hard to hold. A few small additions make them stick:

Examples. A card for a definition is stronger when its back includes a short example.
Context. For vocabulary, a short sentence shows how a word behaves (how to memorize vocabulary).
Images. For things that are visual - an anatomical structure, a diagram, a chart - an image on one side beats a description. In Cirello, images can go on either side of a card and open in a zoomable lightbox.
Your own wording. Explaining an idea in your own words forces understanding. Copying the textbook's sentence into a card does not.
Sources. Wozniak also recommends recording where a fact came from. In Cirello a card can point at a book or website with an instruction like "page 12", so when you doubt a card you can go back to the real thing.

Understand first, then make the card

Spaced repetition keeps what you already understand from fading. It is not a way to understand something. Cards made from material you do not yet grasp turn into rote strings that are hard to use. Read, work an example, make sure you could explain it, and then write the card.

Prefer recall over recognition

A card should ask you to produce an answer, not to pick it from a menu. Producing is the effort that strengthens memory; this is the heart of active recall. A front that already contains the answer in disguise ("Is the capital of Australia Canberra?") tests nothing.

A note on cloze deletion: Wozniak's rules endorse fill-in-the-blank cards. Cirello does not support cloze deletion yet, so with front and back cards, rewrite the blank as a question.

Keep both directions in mind

For some material you need to go both ways. If you must recall a term from its definition and a definition from its term, those are two different retrievals and two cards. If you only need one direction, make only one. Don't double your workload by default.

Fix cards that keep failing

If a card keeps coming back as a miss, diagnose it before you blame your memory:

Is it asking for too much? Split it.
Is it ambiguous? Tighten the question.
Is it too similar to another card? Add a cue that tells them apart, because interference is a classic cause of mixed-up answers.
Is it about something you never really understood? Go back to the source.

A card that stays hard after fixing is fine. Some things are just hard, and the scheduler will keep them in front of you (how FSRS handles difficulty).

Keep the deck selective

Every card you add is a recurring cost, as how many flashcards to review per day explains. Before adding a card, ask whether you want to still know this in six months. If the answer is no, skip it.

A quick checklist

Does it ask for one thing?
Is the answer short and unambiguous?
Do I understand it?
Is there a cue or example if the fact is arbitrary?
Would I want to know this in six months?

If you want somewhere to apply it, the flashcard app overview shows how Cirello handles capture, and the page for remembering book highlights covers source-linked cards.

How to write good flashcards