He was to do this without the help of any written or printed specimens, with nothing in the shape of a grammar or analysis, but merely by oral communication with his Indian instructor, or with other natives, who, however comparatively intelligent, must from the nature of the case have been very imperfect teachers.

He applied himself to the work with great patience and sagacity, carefully acting the differences between the Indian and the English modes of constructing words; and, having once got a clew to this, he pursued every noun and verb he could think of through all possible variations.

In this way he arrived at analyses and rules, which he could apply for himself in a general manner.
Neal says that Eliot was able to speak the language intelligibly after conversing with the Indian servant a few months.

This, in a limited sense, may be true; but he is said to have been engaged two years in the process of learning, before he went to preached to the Indians.

In that time he acquired a somewhat ready facility in the use of that dialect, by means of which he was to carry the instructions of spiritual truth to the men of the forest, though as late as 1649 he still lamented his want of skill in this respect.
Notice having been given of his intention [of instructing the Indians], Mr.Eliot, in company with three others, whose names are not mentioned, having implored the divine blessing on the undertaking, made his first visit to the Indians on the 28th of October, 1646 at a place afterwards called Nonantum; a spot that has the honor of being the first on which a civilized and Christian settlement of Indians was effected within the English colonies of North America.