It's an assembler, which means it's primary use is for programming. However, it can do data very easily, and text happens to be data. To use it, simply open up a command prompt, go to xkas's directory, do
xkas assembly.asm rom.smcand you're good to go. The way I typed it there, both the assembly file and rom need to be in xkas's directory.
To insert text, you do this:
table FF6table.tbl,rtl ; the table you want is FF6's "battle" table which handles everything except dialogue. it is initially labeled "Ff3battl.tbl" at Lord J's site.
DW ptr ; the pointer to change!
DB "I'm text!",$00
I left out the other 254 pointers, which you'll have to also change, as well as a location. But that in a nutshell is basically how you would do this process. You could optionally use atlas, but I'm afraid I haven't used that program.