For 99% of the verses, they could merely pick the translations they liked the best and reword it in the generally cold and consistent "modern" style of the NWT. Only if they came to verses that allowed for a number of different possibilities or a problematic doctrinal issue (mostly Trinity-related), they might resort to the commentaries and dictionaries and other tools they must have had like Gesenius' Hebrew Grammar, Liddell & Scott, etc.
If one were to read through the Old and New Testament as translated in the NWT. It is rather obvious that the translators were more adept at translating the Greek as opposed to the Hebrew. The New Testament is much more readable than the old. I recall a review of the NWT OT by H.H. Rowley back in the early to mid 50's. He criticized the translators wooden literalness. This showed that the translators were not as grammatically strong in their renderings of certain passages. I do recall Genesis 18 being particularly odd reading, amongst other passages. Anyone who has studied Hebrew and read the NWT, can see that the translators did do a fresh translation, although their command of the language was not very strong. Benjamin Kedar, a Hebrew Scholar in Israel, always enjoyed using the NWT, mostly because of its literal renderings. As such, since the early 70's he used it in translation studies he had conducted and printed in scholarly journals. So I sincerely doubt that all they did was pick and choose from other translations.
Having done some work for both Bert Schroeder and George Gangas, I know that Schroeder knew very little about NT Greek and, although Gangas knew and spoke modern Greek, he would be of only a little help to translate koine (NT) Greek. Fred Franz' Greek was much more useful for translation purposes (by translation, I mostly mean making use of the commentaries, dictionaries, etc.)
From those I have interviewed in the past. It seems that George Gangas did have a command of the Koine Greek language, although he often resorted back to modern Greek in his explanations of grammar etc...Bert I was told did have a command of the Greek language upon his graduating from the University of Michigan, however, in later years, he oftened did not use it, and subsequently lost much. He oftened was seen looking up reference works, rather than relying on his own knowledge.
I recall one time interviewing Edward Dunlap before he passed away. He indicated that Fred Franz was solely responsible for the Kingdom Interlinear Translation in 1969. He had two assistants, whom Dunlap could not recall their names, but the translation for the most part was handled completely by Franz. When asked about his opinion regarding Franz' knowledge of the Greek language, he responded similar to Ray Franz' comment in his book "Crisis of Conscience" in that Freddy's knowledge was sufficient enough to attempt a translation of this kind.
After interviewing others at Bethel, I have come to know of several who assisted the NWT committee in subsequent revisions. I heard that individuals like Nelson Herle, John Albu, and a few others outside of bethel assisted with the 1984 revision. I was also told of a Bro. Horowitz who was preparing an interlinear of the old testament. It was apparently going to be prepared and submitted for review by the Governing Body. However a few on the GB didnt take to kindly to his work, and subsequently put some pressure on him to stop working on that project. Also, in recent years more information regarding the NWT has been leaked. Which to me shows that Ray Franz was not privy to the entire story. Many don't know about the assistance to the committee provided by Norman Swift and Karl Klein, as well as some "outside" assistance by individuals like Steven T. Byington.
I am sure much more can be said, and with time the full story will eventually become public knowledge.
Alf3831