By meditation I'm referring to the form commonly associated with the word: the buddhist form - a man sitting cross-legged - as opposed to meditation as a form of prayer (catholic rosary). Though they are both useful psychological tools, they have separate effects and purposes.
The following describes the most common form of meditation (Anapanasati) in its simplest form, this is exactly how I meditate:
I sit cross-legged on the carpeted floor, close my eyes, and take a relaxed, deep breath in, then out.
I am consciously focusing on my breathing, it functions as the anchor.
As I breathe, thoughts will eventually come to the surface, let them.
The thoughts may be neutral, without eliciting an emotional response, or it could be pleasant or unpleasant. Regardless, as the thought arises I do not react to the fact that it arises, I accept that it is there, the only conscious action I take is to slowly, relaxed, return my focus to my breathing.
I do this for as long as I want, aim for whatever you are comfortable with.
Remember, that there is no game taking place, there is no scoreboard, there is no victory condition where you win because you had the fewest thoughts or the most thoughts, a lot of people treat this as doing something and so they focus on that they are "doing" this correctly - but you're not doing, you are in fact not-doing. It stands alone, so it is hard to compare meditation to something else, the closest parallel I can think of in this context is relaxing and its not much better of a metaphor. Nonetheless:
When you relax your body you are not consciously relaxing your body, you are not "doing" relaxation, you are merely relaxed, really you are un-doing tension. Same thing with meditation, if you've never meditated before and you have an active mind you are going to have hundreds of thoughts racing by, you will be absent minded more often than not, you will forget to return to the breath until it eventually occurs to you after a long period of time - this is to be expected, this is okay, you are essentially learning and experiencing an entirely novel thing without any overlap with anything prior. Do not struggle, let go, relax.
Initially, when you first start meditating, depending on your psychological state, you may have a very active mind full of thoughts or you might already have a fairly quiet mind, regardless this simplest form of meditation will overtime permanently give you a quieter mind. By meditating and focusing on your breath, you are able to slowly change the way you respond to these thoughts in your head, this includes images that pop in your head - also a form of thought.
By meditating you put yourself into a position which is essentially the same as exposure therapy, as you train the skill of refocusing your attention by manually returning your focus back to your breath, you are simultaneously exposing yourself to the thoughts that arise within you - which also gives you insight into yourself - some of these are so subtle as to be seemingly unfelt, automatic, and so you may have missed them and the cycles that might be attached to them, when they appeared in your life outside of meditation.
The exposure therapy aspect with meditation is that you are also altering your reaction to your own thoughts, this is the key benefit of meditation that changes lives and the most counterintuitive aspect of the mind. Thoughts arise, we react to them, which culminates in various emotions which lead to action. The thought responsible for triggering its respective cycle is referred to as a core belief. There are also images/flashes which trigger reactions which likewise culminate in emotional responses and actions. Our natural reactions to both of these is to remove ourselves from them, to get away, to distract, to avoid.
This has the unfortunate effect of sustaining the existence of said core belief/image. By reacting to them, we guarantee its continued existence within our mind, the thought, the image will return at some point and we will react to it and it will trigger a cycle. By meditating, by allowing the thought to arise and by calmly, in a relaxed state, accept the thought as a thought and the fact that it is there, and returning our focus back to the breath, we alter the way we react to it, it becomes less potent until it has lost its potency and - seemingly paradoxically - ceases to be and ceases to return.
The timeframe for this varies based on each thought/image's potency, but the solution is the same, and through meditation you place yourself in the best possible position and with the best possible tool to deal with them.
Having meditated almost daily since 2014 nothing I've done has had a more profoundly positive and multifaceted effect. Though I am still impulsive and currently suffering from the physical symptoms of depression due to a vitamin D deficiency (takes awhile to get your levels up with supplements), I used to suffer from severe anxiety, anger issues, emotional disturbances in general, these were all reactive issues tied to thoughts and images and the cycles associated with each. Through meditation, I no longer struggle with any of those, any thought that comes to me is accepted, processed, without reaction - and thoughts that arise from real things, say an interview, the reactions are lessened as I am able to be inside my head and look for the thoughts, I might not catch them when they arise but when I catch myself feeling a certain way I am able to essentially meditate on the spot wherever I am and try to find out what is going on.
This is why I say the effects are permanent, because I don't do sitting meditation often and haven't for a couple of years, my mind is as quiet as it was at the end of the first year of focusing entirely on meditating, getting to the point where I was sitting in full-lotus for an hour a day just meditating. That's the other reward you get from this, you get a useful tool to use in psychological emergencies, namely refocusing your thought to your breathing wherever you are, which calms you - but you also get a mind that is generally more quiet, I used to be plagued by what is referred to as intrusive thoughts - which I believe is terribly named because it gives people the impression that thoughts aren't actually intrusive when that is exactly what all thoughts are, and you get to see that in action the more you meditate.
Nonetheless, as was already said above with meditation as exposure therapy, intrusive thoughts are not separate from thoughts, nor are images that we react to - with what we consider unwanted or negative emotions - separate from other images or thoughts. Everything is a thought, and once you treat it as such - once you through meditation control your reaction - you reduce everything to its proper place as merely a thought in your mind, once reduced, once removed of all potency, it disappears.
Finally, this is not the goal you should have in mind for meditating. Not because it isn't one of many fruits given to you by meditating, but because your focus should never leave meditating for meditation's sake. This returns your focus to the task at hand: breathing, that is all there is to it. The greatest irony of the psychological world is that none of it is real, it is all shadows flickering across the mind, there are no mountains to climb, no demons to fight, no battles, no wars, there is only you - the consciousness inhabiting your body - and the thoughts originating from you, coming and going within your mind. Once you master this, once you use it to overcome whatever issues you have - if you have any that have their source in thought - though you will obviously be happy to have overcome them, looking back you come to realize it was never real to begin with.
There are other specialized forms of meditation falling under guided meditation in that you are either thinking about a specific thing, mantra, sound - or as is more common now, listening to someone talking you through some form of meditation. These do not have any of the benefits gained from standard meditation, as you are no longer aware of your own thoughts, but merely listening or thinking about something in particular.
Do not replace Anapanasati meditation with any of these guided or specialized forms of meditation.
nice try.