Above and beyond the labelconvert documentation page linked above, I can’t really advise on how to generate the necessary lookup table files without going to more effort in describing it than what would have been required to simply generate the file. I can only suggest reading that page multiple times and understanding it as best you can. You could also try modifying one of the files in share/mrtrix3/labelconvert/ (e.g. delete nodes, change values), re-running labelconvert, observing what effect that has on the output image, and see if that helps to make the operation of this command “click”.