No need to experience anattā, but experiencing anatta-lakkhaṇa,non-self-characteristic, is required

Everyone are experiencing anattā (5 aggregates), but only the insight-practitioners are understanding anattā as anattā. So, the practitioner is trying to understand "how it is not attā", then he practice to understand like that in everything. But, he doesn't practice to experience anattā. Actually, he practice to experience anatta-lakkhaṇa of anattā (5 aggregates), which he understanding it as attā, by ñātapariññā. Then he practice, tīraṇa-pariññā, that experience in everything to abandon, pahāna-pariññā, atta-vipallāsa in everything.
The deep concentration-meditation can help the insight-practitioners to understand anattā as anattā easier. But the concentration-meditation can not understand anattā by itself, so we need Buddhā to teach us the insight-meditation; we can't practice the insight-meditation before Buddhā enlightened and teach us insight-meditation.
The understanding in anattā as anattā is gradually practice in neyya-practitioner, such as many bhikkhus who gradually memorize a new "little"-sutta in Saṃyuta-nikāya each time after they finished to practice the previous sutta and still not enlighten; but abruptly practice in ugghaṭitaññū-person and vipacitaññū-person, such as Sāriputta and Moggallāna. So, the gradual or abrupt practice are depend on 5 indriya/bala of each practitiner.
Another, this question is not connect to 4 stage of enlightenment, because the sotāpanna already perfectly abandoned atta-saññā/citta/diṭṭhi-vipallāsa, so every ariya have fully anatta-saññā/citta/diṭṭhi without vipallāsa.
So, the question's explanation should be "How to meditate insight meditation", which is explained in most sutta, especially abhidhamma. For the summary:

1. The practitioner has to pause bodily/verbal/mental unwholesome-mind/mind's factors by concentration-meditation.
2. Then the practitioner has to practice the relativity-understanding of causes-aggregates and their effects-aggregates in paṭiccasamuppāda-cycle into everything, included hell and heaven, past and future, every change. Then after the practitioner understand 3 characteristics, anicca-lakkhaṇa/dukkha-lakkhaṇa/anatta-lakkhaṇa, by that practice, he has to practice to see that 3 characteristics into everything, included hell and heaven, past and future, every change.
3. The tips to accelerate meditation speed is "finding the tipitaka-memorizer who carry on tipitaka-tradition from the the previous tipitaka-memorizer" to be your teacher, because he know many tactic from tipitaka to teach you meditation.

For the meditation in brief or advance, all already taught in tipitaka and atthakathā. Actually, it is better than my summary above, but you have to successfully meditate the insight-meditation at pa-auk with the tipitaka-memorizer. Then, after you enlightened, you will understand tipitaka&atthakathā easily.