Use a standard sample holder and fill it to the top, flattening the LaB6 with a slide. After that you take data over the entire range at a slow speed to get the best statistics. LaB6 has peaks that are resolution limited and so when you fit the data with GSAS2, you do not want to use any broadening parameters for the sample but instead fit the peaks with the instrument parameters only (this is the same as in one of the GSAS2 tutorials for solving a structure and doing a Pawley refinement). You can, however fit the zero shift (not the sample displacement if you have prepared your sample correctly). Once you have all the instrument parameters, these can be written out as an instrument parameter file.

You should occasionally (weekly) run the same LaB6 standard so store it in a safe place and do not take the powder out if you can.

Hi Professor, thank you for your answer, I just followed the direction provided by you in my second question, and finally I found a very nice tutorial about how to create a .prm file for a diffractometer using LaB6 standard.

https://subversion.xray.aps.anl.gov/pyGSAS/Tutorials/CWInstDemo/FindProfParamCW.htm

Thank you!!!