The following is an amalgamation of the various accounts of the 3 Witness’s experience. I believe their “eyes were prepared” by taking an entheogen, namely psilocybin.
The reason why I believe it was psilocybin is because they were not sleeping, which indicates that Amanita was not administered. Additionally, the experience is only ever talked about as having taken place on one day, which indicates that datura was not administered as they would likely be over a 24 hour experience.
You can read many of the original accounts on Fuller Consideration.
1
A. The Golden Plates were not revealed to anyone but Joseph Smith Jr. during the translation process.
B. However, the Plates themselves indicated that once translation was finished three individuals would be chosen to verify their existence.
C. These chosen witnesses would have contributed to the translation effort and, through Divine means, would be granted the opportunity to see the plates themselves, thus confirming their existence firsthand.
D. They would then share their testimony regarding the plates and what was written on them to the world.
E. However, to be granted this opportunity, these individuals would need to show true humility and have a strong faith.
2
A. Leading up to this point, multiple Divine messages suggested that Martin Harris, Oliver Cowdery, and David Whitmer were to be these witnesses.
B. In June 1829 these three men met at the Whitmer family’s farm and approached Joseph Smith Jr., expressing their desire to become the three witnesses and see the Golden Plates for themselves.
C. They were insistent on this matter and pressed Joseph Smith Jr. for an answer. Smith consulted his seer stone for guidance, and it was confirmed that they were to be the three witnesses.
3
A. Several days later on the bright and beautiful afternoon of June 28th, 1829, David Whitmer was busy plowing his field when he was approached by Joseph Smith Jr. and Oliver Cowdery. They asked David to join them while they went to get Martin Harris, and together the 4 of them would view the plates in the woods.
B. Once together and before they set out, Joseph Smith Jr. once more emphasized the importance of being spiritually prepared for what they were about to witness. To achieve this readiness, they all partook in the Sacrament.
C. Following this spiritual preparation, the group ventured into the forest near the Whitmer farm. They found a clearing on a hill, and it was there that they all knelt down and prayed for their witness.
D. However, the awaited vision was slow to appear. Martin Harris began to feel that his presence might be the cause of the delay, so he decided to separate himself from the rest of the group.
4
A. After Harris left they began praying once more, and just minutes later they could see with their spiritual eyes.
B. They found themselves surrounded by an extraordinary light, more brilliant and stunning than fire, which hung above them and spread out far around them.
C. Along with the light they felt a strange and entrancing influence that was so intense it made them feel as though they were rooted to the spot they were in. Yet, this force also filled them with an indescribable sense of absolute joy.
D. An angel descended from the sky, its appearance defying mere words – its form, voice, accent, and choice of words were so heavenly that the angel seemed more like a profound experience than a physical entity.
E. A wooden table materialized a short distance from them, on which the Golden Plates were revealed. The Angel invited them to look closely at the plates and turned the pages to showcase the engravings on them.
F. In addition to the plates they were also shown other relics including the Sword of Laban, the Liahona, the Interpreters, the Brass Plates, the Plates of the Book of Ether, and other records.
G. Although they could see these sacred items, they were not permitted to touch any of them.
H. After viewing these things, the angel emphasized that this experience was made possible through the power of God, and they were obligated to share what they had witnessed with the rest of the world.
5
A. Joseph left David and Oliver to find Martin, who was a good distance away and deeply absorbed in his prayers.
B. Martin sheepishly admitted to Joseph that he hadn’t received his witness yet, and asked Joseph to pray with him so that he might receive it.
C. Before their prayer was even finished, the same Divine vision was granted to Martin Harris.
D. Overwhelmed with immense joy, Martin leaped into the air, exclaiming “Hosanna! Bless God! It’s enough, it’s enough; my eyes have seen it, my eyes have seen it!”
6
A. These three witnesses were able to receive the experience that they sought.
B. Now they were to carry out the directive they had been given, which was to share their experiences that they had.