I'm not sure I understand your question. If the fid is produced by varian software, then it will already have the necessary headers (note the plural) prepended to the data. Ie data is from another brand of spectrometer, I believe Varian has utilities to convert the data to their format, complete with the necessary headers. If the fid is from another source (say a simulated or synthetic fid) then you will have to pre-pend a file header to the entire file, and a block header to each block of data. These are well described in the Varian user preogramming manual. If necessary, I can send you class definitions that describe the headers. These are used by SpinWorks to read Varian data but could also be used to write Varian compatible data if you have some programming experience.