Good Reference: Shu's Blog (in Japanese)

1. Download ipsw for iPhone6s iOS 11.3.1. https://ipsw.me/download/iPhone8,1/15E302
   • check checksum. $ md5sum iPhone_4.7_11.3.1_15E302_Restore.ipsw
2. Make an .ipcc file.
   1. unarchive .ipsw file by 7-zip.
   2. open the .dmg file by TransMac. (Free for 15 days)
   3. Go to /System/Library/CarrierBundles/iPhone/Docomo_jp.bundles.
   4. copy all the files to Payload/Unknown.Bundle/
   5. make .zip archive.
   6. rename it to .ipcc.
3. Inject the .ipcc file to your iPhone through iTunes.