Codex Sinaiticus Exposed Part 3: Sinaiticus was written in the monastery of Panteleemon in Mount Athos, Greece.

Here are the details from Simonides letter to the newspaper printed & published in The Journal of Sacred Literature 1862-10: Volume 2 Issue 3 [October, 1862]:

"As what Dr. Tregelles calls my ‘story’ has never been published, and as that gentleman can only have heard of it through an indirect medium, it may interest both Dr. Tregelles and your readers to have the ‘story’ direct from myself. I will tell it as briefly as possible. ... About the end of the year 1839, the venerable Benedict, my uncle, spiritual head of the monastery of the 
holy martyr, Panteleemon, in Mount Athos, wished to present to the Emperor Nicholas I., of Russia, some gift from the sacred mountain, in grateful acknowledgment of the presents which had, from time to time, been offered to the monastery of the martyr." (The Journal of Sacred Literature 1862-10: Volume 2 Issue 3, Page 248 Link).

First, Simonides' uncle Benedict, was the monastery of Panteleemon's head [meaning their leader, not the physical head of a person] and wanted to give Russian Czar Nicholas I a gift from the Panteleemon monastery in return for Nicholas' previous gifts. (Remember this detail as this will be an important part on why Codex Sinaiticus was made). Let's continue:

"Dionysius, the professional caligrapher of the monastery, was then begged to undertake the work, but he declined, saying that the task being exceedingly difficult, he would rather not do so. In consequence of this, I myself determined to begin the work, especially as my revered uncle seemed earnestly to wish it." (The Journal of Sacred Literature 1862-10: Vol 2 Iss 3, Page 248 Link).

Before Simonides wanted to write Sinaiticus in 1839, he studied paleography and archeology and learned, on how to prepare and write out manuscript skins and writing inks that looked ancient, from his uncle Benedict for 14 months from 1837 to sometime in 1838.

"I also visited Mount Athos in 1837, in which year the discovery of the library took place. I remained fourteen months at Mount Athos increasing my theological knowledge under my uncle at the same time studying, scientifically, paleography and archaeology.
'' I was taught the means of knowing the ancient MSS. of every period anil of every nation, their changes from time, also the knowledge of the skins, and the chemical preparation of the different writing-inks and the effects of the atmospheric changes of the different climates of the world. Further, I acquired the knowledge of the preparation of the skins of every city of the ancient nations and such other information as is requisite with regard to the indisputable evidence both of the spuriousness and genuineness of MSS. of every kind. (Letter of C. Simonides, The Journal of Sacred Literature and Biblical Record, April 1863 Link)

Next we will see that Simonades, in 1839, after deciding to write Sinaiticus, chose an almost completely blank volume (book) that had written pages in it from centuries ago, took out those written pages, and then started to write out the Old and New Tetsaments and the spurious, unbiblical and non-inspired "Epistle of Barnabas" and "pastoral writings of Hermas": "I selected from the library of the monastery, with Benedict’s permission, a very bulky volume, antiquely bound, and almost entirely blank, the parchment of which was remarkably clean, and beautifully finished. This had been prepared apparently many centuries ago—probably by the writer or by the principal of the monastery, as it bore the inscription, EKAOTION MANHIYPIKON (a Collection of Panegyrics), and also a short discourse much injured by time.“I therefore took possession of this book, and prepared it by taking out the leaf containing the discourse, and by removing several others injured by time and moths, after which I began my task.