mkdir graphapp # make a directory for it
mv GraphApp.Athena.tar.gz graphapp # move the tar file there
cd graphapp # go into the directory
gunzip GraphApp.Athena.tar.gz # unzip the file
tar xf GraphApp.Athena.tar # untar the file
This will produce a directory called something like
GraphApp_Athena_Src_2_45 which will contain several .c and .h files,
as well as the COPYLIB.TXT file and a Makefile.
Having made any changes to the Makefile that need to be made, you should now type:
make pizza
to see if all is going well.