You will also have to get your observed structure factors into HKL format. This process is more difficult because there are so many different formats which the many data collection software programs produce. There is no general purpose format conversion program for reformatting structure factor files supplied in TNT. You will have to write a program to convert whatever format you have into TNT's or find an existing program elsewhere. The format is described in Appendix A of the TNT Reference Manual.
These days most diffraction data sets are reduced with the programs Denzo or Mosfilm. The output of either can easily be converted to a .mtz format file. The CCP4 program Truncate can be used to convert from intensities to amplitudes and then the program mtz2various will write a basic TNT hkl format file.
There are two difficult points which remain. The asymmetric unit of data presented to TNT programs must conform to their expectations and the reflections must be supplied in the proper order. If some of your reflections fall outside of the box defined for your space group they will be ignored. No error will be issued.
Once you have your set of Fobs written in the TNT HKL format you can use the correct command to fix these last two problems.
When the conversion is complete the next step is to add remarks (REM cards) to the top of the file. Every HKL file should include remarks which describe the molecule, who collected the data, how it was collected, when it was collected, and other relevant information like cell constants and resolution limits.