Making correct use of impressions is a matter of not assenting to false value-judgments.
"The key to transforming oneself into the Stoic sophos (wise person) is to learn what is ‘in one’s power’, and this is ‘the correct use of impressions’ (phantasiai), which in outline involves not judging as good or bad anything that appears to one. For the only thing that is good is acting virtuously (that is, motivated by virtue), and the only thing that is bad is the opposite, acting viciously (that is, motivated by vice)" (Seddon, Epictetus' Handbook, pg. 3).
"It is not the things themselves that disturb men, but their judgements about these things. For example, death is nothing dreadful, or else Socrates too would have thought so, but the judgement that death is dreadful, this is the dreadful thing. When, therefore, we are hindered, or disturbed, or grieved, let us never blame anyone but ourselves, that means, our own judgements. It is the part of an uneducated person to blame others where he himself fares ill; to blame himself is the part of one whose education has begun; to blame neither another nor his own self is the part of one whose education is already complete" (Epictetus, Encheiridion 5; Oldfather).
"When you see someone weeping in sorrow, either because a child has gone on a journey, or because he has lost his property, beware that you be not carried away by the impression that the man is in the midst of external ills, but straightway keep before you this thought: "It is not what has happened that distresses this man (for it does not distress another), but his judgement about it." Do not, however, hesitate to sympathize with him so far as words go, and, if occasion offers, even to groan with him; but be careful not to groan also in the centre of your being" (Epictetus, Encheiridion 16; Oldfather).
"Bear in mind that it is not the man who reviles or strikes you that insults you, but it is your judgement that these men are insulting you. Therefore, when someone irritates you, be assured that it is your own opinion which has irritated you. And so make it your first endeavour not to be carried away by the external impression; for if once you gain time and delay, you will more easily become master of yourself" (Epictetus, Encheiridion 20; Oldfather).
"It is chiefiy with this principle in mind that a man must exercise himself. Go out of the house at early dawn, and no matter whom you see or whom you hear, examine him and then answer as you would to a question. What did you see? A handsome man or a handsome woman? Apply your rule. Is it outside the province of the moral purpose, or inside? Outside. Away with it. What did you see? A man in grief over the death of his child? Apply your rule. Death lies outside the province of the moral purpose. Out of the way with it. Did a Consul meet you? Apply your rule. What sort of thing is a consulship? Outside the province of the moral purpose, or inside? Outside. Away with it, too, it does not meet the test; throw it away, it does not concern you. If we had kept doing this and had exercised ourselves from dawn till dark with this principle in mind,—by the gods, something would have been achieved! But as it is, we are caught gaping straightway at every external impression that comes along, and we wake up a little only during the lecture, if indeed we do so even then. After that is over we go out, and if we see a man in grief, we say, "It is all over with him"; if we see a Consul, we say, "Happy man"; if we see an exile, "Poor fellow"; or a poverty-stricken person, "Wretched man, he has nothing with which to get a bite to eat." These, then, are the vicious judgements which we ought to eradicate; this is the subject upon which we ought to concentrate our efforts. Why, what is weeping and sighing? A judgement. What is misfortune? A judgement. What are strife, disagreement, faultfinding, accusing, impiety, foolishness? They are all judgements, and that, too, judgements about things that lie outside the province of moral purpose, assumed to be good or evil. Let a man but transfer his judgements to matters that lie within the province of the moral purpose, and I guarantee that he will be steadfast, whatever be the state of things about him" (Epictetus, Discourses, 3.3.14--19; Oldfather).
"But as it is, we are fiery and fluent in the schoolroom, and if some trivial question about one of these points comes up, we are able to pursue the logical consequences; yet drag us into practical application, and you will find us miserable shipwrecked mariners. Let a disturbing thought come to us and you will find out what we have been practising and for what we have been training! As a result, because of our lack of practice, we are ever going out of our way to heap up terrors and to make them out greater than they actually are. For example, whenever I go to sea, on gazing down into the deep or looking around upon the expanse of waters and seeing no land, I am beside myself, fancying that if I am wrecked I shall have to swallow this whole expanse of waters; but it does not occur to me that three pints are enough. What is it, then, that disturbs me? The expanse of sea? No, but my judgement. Again, when there is an earthquake, I fancy that the whole city is going to fall upon me; what, is not a little stone enough to knock my brains out?
"What, then, are the things that weigh upon us and drive us out of our senses? Why, what else but our judgements? For when a man goes hence abandoning the comrades, the places, and the social relations to which he is accustomed, what else is the burden that is weighing him down but a judgement? Children, indeed, when they cry a little because their nurse has left, forget their troubles as soon as they get a cookie. Would you, therefore, have us resemble children? No, by Zeus! For I claim that we should be influenced in this way, not by a cookie, but by true judgements" (Epictetus, Discourses, 2.16.20--6; Oldfather).
"There came in to visit Epictetus one day a man who was on his way to Rome, where he was engaged in a lawsuit involving an honour to be bestowed on him.[1] Epictetus asked what the reason was for the trip to the Capital, and the man proceeded to ask what opinion he had about the matter. If you ask me what you are going to do in Rome, says Epictetus, whether you will succeed or fail, I have no precept to offer. If, however, you ask how you are going to fare, I have this to say: If you have sound judgements, you will fare well; if unsound judgements, ill; since in every case the way a man fares is determined by his judgement. For what is it that made you eager to be elected patron of the people of Cnossos? Your judgement. What is it that impels you now to go up to Rome? Your judgement. And that in stormy weather, in danger, and at expense?—Yes, but I have to.—Who tells you that? Your judgement. Very well, then, if a man's judgements determine everything, and if a man has unsound judgements, whatever be the cause such also will be the consequence. Do we all, then, have sound judgements, both you and your opponent? If so, then how do you come to disagree? But do you have sound judgements rather than he? Why? You think so. So does he, and so do madmen. This is a poor criterion. But show me that you have made any study of your own judgements and have paid attention to them. And as now you are sailing to Rome so as to become patron of the men of Cnossos, and you are not satisfied to stay at home and keep the honours which you had, but you have set your heart upon something greater and more conspicuous, so did you ever make a voyage for the purpose of studying your own judgements, and of rejecting one, if it is unsound? Whom have you ever visited for this purpose? What time have you set yourself, what period of your life? Review the periods of your life, all to yourself, if you are ashamed to do so before me. When you were a boy were you in the habit of examining your judgements? Did you not habitually do what you then did just as you do everything now? And when you grew to be a youth and were attending the lectures of the rhetoricians, and were yourself practising, what did you fancy that you yet lacked? And when you were a young man and began to take part in politics, and to plead cases yourself, and to have a good reputation, who any longer seemed in your eyes to be your equal? Would you under any circumstances have submitted to be put through an examination on the charge that you had wretched judgements?" (Epictetus, Discourses, 3.9.2--9; Oldfather).