illustration (attribution, if any possible, is at the end of the article)
A common mistake on the spiritual path is to separate yourself from your potential (i.e. your "buddha-nature"). It typically goes by considering <something> "applies to Enlightened beings" only…
The Buddhist path is eminently practical.
One difficulty is that nirvāṇa is unconditioned.
There's no path predetermined for you to reach the goal of holy life, primarily because your circumstances and conditions are unique: nobody else could have trodden before you the path you need to follow. They walked their own path…
However, this does not mean the teachings are pointless: the teachings help you remove barriers, they provide a map, they help you build the appropriate vehicle… Clearly, having a raft is useful to cross to the other shore (even if you don't cross exactly the same river as others, or at the same point of the stream!), just like tearing veils helps to see and opening doors helps crossing thresholds! But possessing a raft is not the same as using it, opening a door is not the same as walking through, and tearing veils is not the same as looking up and observing! So no matter how much help you get from the path, nirvāṇa remains 'unconditioned': at the end of the journey, it is about you looking at things as they are and embodying wisdom… and until you do so, the help you receive may increase your potential but not your realisation.
A consequence of the above is that one should not confuse the path with the goal: preparing forever to cross will never get you to the other side, hence practice requires you to trust your potential and actually do it here&now, not postponing forever "until you're ready"!
Of course, maybe you're not ready and you might make mistakes trying… but that's how we learn! As long as you look, as long as you don't build narratives about why failures are due to external circumstances (as if you had nothing to do with picking —or riding— them), as long as you learn lessons, you increase your potential.
Postponing the journey may be smart if you know nothing of the destination but is pretty much a waste of time and of opportunities once you have 'enough' information: the fact of life is you will never be sure and since the world is changing there's always more to know. If you wait to know it all, you'll never make your journey!
Prerequisite to training
Once you understand that training properly requires some initial knowledge about the do's-and-don't's, and may benefit from the availability of coaching, but nonetheless relies at its core on 'doing' rather than 'planning', you can walk the buddhist path…
This requires refraining from separating yourself from the goal, refraining from arbitrarily limiting some wholesome <something> to "Enlightened beings only."
The more you appropriate the qualities of a buddha (Mahayana schools would say "the more you appropriate the buddha-mind"), the more you are a buddha! A buddha is distinguished from others by manifestations of wisdom, compassion, wholesome views, etc. The more you embody such manifestations, the more you are a buddha: there is no self, so what you embody or manifest is who 'you' are!
Mahāyāna schools would say you have "buddha-nature", early schools would not use such vocabulary but they would insist on the reachability of the goal of holy life through "right effort"… All schools basically are clear that if you manifest as a buddha (via speech, action, ethics, intention, wisdom…), you are a buddha: there's no self, only freedom and aggregates of more or less wholesome tendencies (manifested by 'someone' embodying them).
How to train?
A competitive runner trains by running… (s)he might perform other complementary exercises, often to counter-act the danger of extremes and get a more 'balanced' approach, but ultimately (s)he becomes a runner by running! Similarly, a martial artist spends more time practicing martial arts than e.g. classical 'Ikebana' flower arrangements…
There's nothing wrong with any discipline but, to develop and reach one's potential in a particular discipline, you have to practise this very discipline, not another!
Buddhism clearly states that concentration is useful to train in, but that insight (i.e. looking up and observing!) is the key to "seeing things as they are."
This was probably one of the most important insights of the Buddha, and this is the reason why he was dissatisfied with his two initial teachers (who reached the highest levels of concentration —clearly described similarly to the jhānas— but who also clearly didn't find, according to the Buddha, the cessation of saṃsāra).
So how do you train to "see things as they are"?
Do you just say "this is for Enlightened people" and basically forever postpone practising the discipline you had set as your goal?
Or do you try here&now, and again —looking up, observing, noticing when you err in mental proliferations (and how this manifests and what consequences unfold, etc.), coming back to here&now and trying once more?
The reason why concentration meditation is seen as useful training to develop insight meditation is the training into "coming back to the present".
In concentration meditation, the 'present' is voluntarily limited to a particular phenomena —e.g. breathing— in order to facilitate the detection when the mind errs: any other object of thought can be labelled as erring and a signal to come back to the present.
By contrast, insight meditation takes an 'inclusive' perspective on what the present is, but one still needs not to err in the past or the future or in mental models and proliferations full of narratives and speculations about what's not directly perceived.
As such, training in concentration might prove useful to develop the 'vigilance' (the ability to detect when the mind errs), and vigilance is useful for discriminating insights (an appropriate unbiased response to here&now) from mere narratives (at best, an extension of here&now into a narrative with past and future… at worst, utter confusion about what reality is). But training in concentration is not what you want to practise most.
Buddhism is not asceticism: it is unlikely you should become an ascetic to actualise the attainment of the highest goal. For some people, it will be their particular circumstances, but not in general.
Concentration, meditation, ethics are all complementary trainings, just like aerobic training can be useful to any sportsman but doesn't become the core of their specific discipline. The core discipline of Buddhism (and Mahāyāna makes this particularly explicit in relation to the pāramitās) is Wisdom, the rest is useful but is of conditioned nature, dependent on your circumstances, on the situation at hand…
Wisdom manifests as the 'appropriateness' between your response and the circumstances that elicited a response…
How do you train?
By "not knowing", i.e. by not assuming that you know how things will unfold merely because a buddhist teaching said this was "the right thing to do" or other certainty from whatever 'source'.
By following up, by looking at the consequences and responding to this new here&now arising, by checking that the side-effects of some medicine are not worse than what it contributes to manage, i.e. by looking and continuing the engagement, by paying attention to here&now (i.e. 'constant' attention, as in "constantly renewed").
Looking and looking again, not to make up excuses or narratives or build prejudices and half-baked conclusions, but to precisely counter all this: looking and looking again, to adapt, to notice exceptions to the rules, to learn, to respond appropriately even to the rare cases or to what's simply new: life itself in what makes here&now different from all other times and places!
Appropriate wisdom! Give life to wisdom! Look, creatively engage (don't 'plan' to engage), respond with the best of your ability to what the situation requires…
If you get it wrong? Try again! The path is meant to be travelled in actual terms!