There's an entire chapter on this in Hands-On Chaos Magic by Andrieh Vitimus, called "Why is trance needed in magic?" Here's an excerpt:
The fact is, the psychic censor (or the host of other names it is called) completely limits our conception of what is possible. When you believe something is not possible, it is not. [...] If we cannot automatically shift our beliefs and patterns, how can we make more adaptive changes? Both in the internal world and the external world, we are told "no change." Essentially, our conditioning and the near-conscious parts of the mind are working against us when we try to do magic.
What this means is that, for magic to work, you have to allow your subconscious mind to take over for a bit. In order to do that, you have to switch your conscious mind off, or at least bring it to a different state of awareness. This is becuase magic is performed primarily on the subconscious level. Intentions are strongest when supported by the subconscious mind. The subconscious mind also supports your Willing Suspension of Disbelief — in order for your magic to work, you have to actually believe that it will work, without your conscious mind shutting it down at every turn.
I do this to myself all the time, and not just with magic-related stuff. I relentlessly shut myself down with everything related to my writing. I don't let my "muse" generate raw ideas. "No, that's too cliche. No, that wouldn't work. How do I actually write that, though? You're going to have to meticulously plan out that plotline, because as it stands, it's completely nonsensical. There's nowhere to put this dialogue. You're going to have to find something to actually do with that cool magical item, or else it's just a useless MacGuffin and you have a useless predictable plot. The words sound wrong. The words are wrong." I censor my random generation of ideas to the point where I don't actually get any work done.
You have to be able to turn that internal censor off, or you won't actually get any work done. I still really struggle with it.
So, how do you do that? The goal is to put yourself somewhere between light daydreaming and sleep. There are two main types of trance. The first is inhibitory trance, which involves limiting your body and mind in some particular way, intended to quiet it down until nothing exists but the intention or idea being focused on. The other type is excitatory trance, which is an attempt to overstimulate the mind and body until the conscience and the body just shut off. There are actually a lot of different ways of achieving trance, which I didn't know at first. Here's a few:
Sitting or lying still. This is a traditional method of achieving trance, with the ideal being to put the mind somewhere between waking and sleeping. I cannot do this, but it seems to work for a lot of people, especially when accompanied with lulling music, atmosphere, or visualizations.Tai-chi, yoga, or other slow movement. For some people, doing a series of slow movements or holding positions can help them to let their minds wander.Chanting. Chanting a mantra while doing one of the above can help the mind settle into a steady focus on one, singular thing — a desire, a memory, an emotion, a deity or spirit, a sensation, etc. An excitatory form of chanting can be shouting, screaming incoherently, or glossolalia.Scrying. Scrying is a form of divination that involves staring into a blank object, traditionally a crystal ball or a black scrying mirror. One could technically scry any blank surface, though, even just the darkness of a room. You gaze into it until your mind slowly switches off and you enter a dreamlike state.Ecstatic dance or other fast movement. Dancing ecstatically can put the mind in a state similar to a high. It also tires out the body, so if you dance or whirl and then quickly fall to the ground, you'll be too dizzy and euphoric for your conscious mind to be in control.Oscillation. A repetitive swinging or spinning motion can help to both stimulate the body and lull the mind. I usually meditate by pacing back and forth, with the speed of my step reflecting the speed of my thoughts, ranging anywhere from slow walking to fast running and leaping. My conscious mind doesn't really shut off, but my mind certainly wanders.Masturbation. This is definitely one of the easier methods of attaining trance, and it's ideal for Shadow work.Substance-induced trance. Shamans have been using different natural drugs as gateways to powerful trance states for thousands of years. The Dionysian Mysteries use wine as a means of achieving trance, alongside ecstatic dancing. Substance-induced trance is certainly easy, but it's obviously dangerous, hard to control, and not always healthy (to put it mildly). Don't rely on drugs as a means of achieving trance, and if you're going to use them, be as careful and well-informed as possible. Do not attempt except with an experienced practitioner.
How do you know when you've attained it? I actually don't know. I think that depends entirely on the person. It took me ages to realize that I'm actually really good at meditation, because inhibitive forms of achieving trance really don't work for me. I encourage you to find a method of trance that works well for you, personally. Even if it feels like you're not doing it "right." What matters is that you get results. Don't feel like you need to spend thirty minutes quieting down your mind unless it actually helps you.