1) First, from the directory that the core file is in, find the name of the executable which created the core file buy running:
file <core file>
2) Once you find the executable run:
gdb <path to executable> <path to core>
3) Copy and paste the output from the procedure above and provide this with the core file.
Often core files will be large but can be compressed to a much smaller size by running:
bzip2 <core file>
This will result in a file with the same name as the original, with an added .bz2 extension.