It's actually very straightforward:
1. Get an original Japanese ROM (MD5 checksum is f0e3027b9e97618732b4f2d4298ad0cf) , make a copy into a temp folder
2. Put both .xdelta files from their original posts to the same folder
3. Download XdeltaUI (
http://www.romhacking.net/utilities/598/) & extract it there too
4. Run XdeltaUI, select arm7.xdelta in the first prompt, an original ROM in the second & your desired output file (i.e. 'Ni No Kuni.nds')
5. Click the 'Patch' button & wait till the job is done.
6. Repeat step 4, but now in first promit you should select the English_banner.xdelta & as the output from the step 4 as a source ROM. The resulting file should have a new name (e.g. 'Ni no Kuni - The Jet-Black Mage.nds').
7. Click 'Patch' & when it's done put the file on your 3DS' SD card into the \roms\nds directory.
That's it, now run Twilight Menu++ & enjoy.