Choose Discover options for the game you are in
Choose the Discover card that solves your main problem. Check its cost and how the effect will use it.
Pick a job first
Discover offers cards from a pool set by the effect. Before choosing, ask what you need: safety, pressure, more cards, an answer, or a way to finish.
A strong late-game card is no help if you die before playing it. A small removal spell may be best because it saves your hero or protects your board.
When you are ahead, a useful backup may be better than one more threat that loses to the same clear.
Check where the card goes
Does the effect put it in your hand, summon it, cast it, or put it somewhere else? Read the full text.
Check hand space, board space, Mana, and cost cuts. These can change which choice you can use.
If an enemy minion will kill you, an answer now is often better than a large threat later. If you already have a safe answer and need a way to win, the threat may be better.
Know the limits of the pool
Discover pools can depend on the card, class, format, and special rules. Do not assume every Discover can find every card that seems to fit. Do not assume all outcomes always have the same chance.
If your plan needs one exact card, check whether the effect can find it. Until it appears, it is still a risk.
Compare later turns too
Two choices may solve the same problem. A cheaper one may leave Mana for a minion. A broader answer may help against more future threats. Pick for the job it does, not just its size or name.