The basics, it is widely agreed, are what are known as "set" and "setting". That is, what your mindset (or mind state, or where you are emotionally, what your expectations are, etc) and where you are physically and who and what are around you. Try to make those positive, supportive, friendly, stress-free, and comfortable.
It's also important to have an experienced, trusted sitter or guide, and it helps to have a constructive intention for the trip, to be in a safe, supportive environment, not to have any commitments about what you have to do or where you have to be that day (and perhaps the next as well, for recovery). It might help to have some personally meaningful items with you to review during the trip. Things like photographs of loved ones, or other things that remind you of important people or things in your life. It can help to have a good mix of music to listen to.
When all of these things, and perhaps some others not mentioned here, are in alignment, it's more likely that you'll have a constructive trip. But there are never any guarantees (with this or anything else in life, but especially in regards to what is likely to be a very emotionally powerful experience where you might be confronted with all sorts of things that could change your perspective or set you face-to-face with maybe something you weren't ready to face).
The Psychedelic Explorer's Guide[1] and The Secret Chief Revealed[2]. The latter was written by a therapist who conducted hundreds of therapy sessions with MDMA.
[1] - https://www.amazon.com/Psychedelic-Explorers-Guide-Therapeut...
[2] - https://www.amazon.com/Secret-Chief-Revealed-Myron-Stolaroff...
Short answer: Yes, psychedelics including acid can be used correctly.
Psilocybin (the active ingredient in magic mushrooms) has a very similar, albeit a few hours shorter, effect to acid, and it is currently in FDA Phase 2 trials for emotional distress related to cancer, and is likely to soon progress to FDA Phase 3 trials. Some anxiety will occur in some patients, but true "bad trips" where the experience is overall negative seem highly avoidable with the correct procedure.
https://tripsafe.org/acid/#5-bad-trips-do-happen-though-poss...
> A 1955 paper found that anxiety occured in 9% more people who took 25-75 micrograms of LSD, and 14% more people who took 100-225 micrograms of LSD, compared with the placebo group.2
https://tripsafe.org/lsd/#what-is-the-probability-of-having-...
> At a dosage of 67 micrograms17 in a therapeutic clinical environment, we might expect 0% of people to experience some extreme fear,18 for an LSD-adjusted average of around ~2 minutes of strong anxiety19, and we might expect around ~45% to report a persisting positive increase in mood from this dosage assuming they had first tried 35 micrograms, then 67
The last point is particularly good. An expected average of 2 minutes of strong anxiety. With highly compelling therapeutic upsides.
But I think if there wasn't the stigma legal or otherwise, the use of these substances would be much safer. An experienced guide like a sitter or a shaman can guide people through rough experiences and encourage the uplifting ones.
There's a very interesting story about Nancy, an incommunicative, self-destructive, schizophrenic, dying eleven-year-old girl who was administered a high dose of LSD in the 60's.[1][2][3] You'd think she might not be the best candidate for a strong psychedelic experience. But it reportedly saved her life, and helped restore her to sanity. There have been many other cases of psychedelics being used to treat mentally ill people, with impressive results.
Some people do react negatively, but in those cases I really wonder which drug(s) they took, what the dosage was, what their mindset was, what environment they took it in, who they took it with, what they did during the trip, and what intention they had. I would be willing to bet that in most cases many of these factors were far from ideal, and if they'd used them more constructively they would have probably had a better experience.
I do believe it's possible to abuse these drugs, mostly by not taking them seriously enough and not treating them with enough respect, taking them only to "party" or even with destructive intent. And, yes, maybe some people just won't be able to handle psychedelics no matter what. Maybe.
[1] - https://bpaste.net/raw/c86b011161ca
http://reset.me/personal-story/personal-story-ayahuasca-addi...
Alcohol even presents similar issues. If you're in a bad mood, drinking can make you angry and violent. If you're in a good mood it can make you relax. There's definitely a more amplified effect in the case of acid, but it's the same principle.
As Humphry Osmond famously said, "To fathom Hell or soar angelic, just take a pinch of psychedelic." The problem is, you don't know which you'll be doing in a given trip, and it's not uncommon to do both in the same trip, even bouncing from one to the other many times. There are many, many factors that play in to this, some of which are mentioned elsewhere in this thread.
Now, take any 4 weekends from the past six months and compare them, what was on your mind for each Saturday and Sunday? Count the things you might think about in a span of time for any day off.
Now, imagine that thrown into the middle of that, your friend shows you, in the palm of their hand, two tabs of acid.
Add psychedelic state to all your shifts in mood, stream of consciousness and common distractions, and dredge up the baggage of memories reaching back into your childhood, and include existential questions about what it means to exist.
This was also a "new trend" in the early 90's.
If it's unsafe in some cases that will come out in clinical trials just like with any drug.
I'm not saying that all trips are 'actually good' or some such. But it's not like you're rolling the dice; pretty much every 'bad' experience can be attributed to doing too much and/or doing it in a bad setting or with a poor mindset. And even in those cases, some basic prep to learn to not resist the trip, to just go with it, can make recovery quite achievable.
Basically, some of the more 'difficult' trips can also be among the most valuable. To use acid 'correctly' (I disagree with this idea), you simply have to be prepared for whatever comes your way. You may confront some personal demons, but if you're ready to face that, then the benefits from doing so are enormous.
Since the effect is mind altering you can maybe understand how a _very_ altered mind is too different from your normal state for you to remember what it was like being high.
Also a mind-state is affected by context and the context never stays the same, which means there will be some variance.
However I'd argue that the question "can you use <psychadelic> correctly?" makes no sense since the word 'correctly' implies a frame of reference while the psychadelic will force you out of any such frame (depending on dose).
Historically there were complex rituals for "correct" psychadelic use, all heavily religious of course. Our society is not willing to let something like that into the mainstream.
Edit: Sometimes "correct" usage of a drug is to not ingest it, for example cyanide :)