Once you figure out what each Detective saw, index into the person and weapon they saw using the indexes given by their location on the note, and put the resulting letters in the filing cabinet corresponding to each Detective's number. To help in your deduction, remember that anybody between the person who asked for evidence and the person who provided it cannot have seen any of the three things asked about.
Looking at the two questions for which we know both the detective who asked and who answered, we can eliminate all but one of the detectives from being in the Library, which means the remaining detective must have been the one in the Library.
Using Detective 6's question, we know Detective 3 could not have talked to Mrs. Peacock, which means according to Detective 2's question, Detective 3 also could not have seen the Candlestick. Thus, we know who saw the Candlestick, and from that, we know both who talked to Mr. Green and who talked to Mrs. Peacock.
Now that we know who Detectives 3 and 4 talked to, we also know who must have responded to Detective 1's first question. That means we know who talked to Colonel Mustard and where Colonel Mustard was.
Now using the questions of Detective 3 and Detective 5 together, we can deduce who talked to Mrs. White and Miss Scarlet. Note that for both, Miss Scarlet must come after Mrs. White in turn order from the detective who asked the question or else the person who talked to Miss Scarlet would have spoken up first. Also remember that nobody can answer their own question! By process of elimination, we know who talked to Professor Plum, too.
From there, we can figure out Mrs. White's weapon, as well as where she was. Then, only one Detective left could have been in the Kitchen, meaning they also saw the Knife. Process of elimination gives the Detective that saw each remaining weapon and subsequently their locations, finishing the puzzle.