The skill is the conversation, not the cards
You can know all 78 cards and give a bad reading to another person, because reading for someone else adds a layer that self-reading does not have: everything you say lands on a person who did not choose the words. The interpretive work is roughly the same. The delivery is an entirely separate skill and it is the one that determines whether the reading helps.
Most people who feel they are bad at reading for others are not misreading the cards. They are narrating an internal process out loud without editing it, which is why it comes out as a stream of possibilities and hedges.
Set the question up before you touch the deck
Ask what they want to look at, then ask a second question about the first one. "Should I take the job" almost always has something underneath it, and two minutes of conversation before the shuffle will produce a better reading than any spread choice. Write the final question down and read it back to them.
This is also where you set expectations, briefly and without a speech: the cards describe the situation rather than predict outcomes, and nothing here is a verdict. Saying it once at the start prevents most of the difficult moments later.
What to do when a hard card lands
Do not soften it into meaninglessness and do not deliver it as a pronouncement. Describe the card plainly, then hand it back as a question: "The Three of Swords is here in the position about what is currently going on: does that connect to anything?" The querent almost always knows, and letting them say it is both more accurate and considerably kinder than telling them.
The Tower, Death, the Ten of Swords and the Three of Swords are the cards beginners most want to skip past. Skipping them is the one thing that reliably breaks trust, because the person can see the card too.
Questions about people who are not there
You will be asked what someone else is thinking or feeling. The honest handling is to reframe it out loud: the cards describe the situation from the querent's side, not the other person's interior. "What is the shape of this from where you are standing" is answerable; "does he love me" is not, and pretending otherwise is where readings do actual harm.
The same applies to reading about a third party who has not consented, a partner's fidelity, a colleague's intentions. You can decline, and a short "I can read your side of this, not theirs" is enough.
Where to stop
Do not read on medical, legal, financial, or psychiatric questions. Not because the cards would say something dangerous, but because a person in distress may act on what you say, and none of those domains is one where a randomised prompt should be influencing a decision. The correct response is to say you do not read on that and to mean it.
End the reading rather than letting it trail off. A short summary (what the cards described, and the one thing worth sitting with) gives the person something to leave with, and stops the session from expanding into a series of clarifying cards nobody planned.