We went to the home office in croydan today and was pleasently surprised by the process. We were sucessful in obtaining my husbands spouse visa

However, it states on the letter about applying for idefinate leave to remain in the UK 28 days before his spouse visa expires. Does anyone know what we need to start collecting now in order for him to obtain idefinate leave to remain?
Any suggestions would be greatly appreciated.
Kind Regards
Yasmin