I'm not sure this is either the best nor the only method, but it does have its advantages:
"I took the Gaelic Bible which, from my previous acquaintance with the English, I soon learnt to read. A Gaelic Grammar helped me over the difficulties of flexion. Southey, I remember, somewhere in his diary says, 'that it was his fashion always to commence the study of a new langauge with a version of the New Testament,' and there can be no doubt that to those who know their Bibles there can be no better method proposed. The language of both Gaelic and English versions is classic, and about the best to be had. Let the student read the Gaelic Bible daily, along with the English, and translate the one back into the other alternatively, and this will be a hundred times more efficient than any other method, and will work the language into his head."
- MacLaren, J. MacLaren's Gaelic Self-Taught Gairm: Glasgow, 1992. p168-9