JSON (Java Script Object Notation) is same like "XML" formats and other related formats which compress or reduce size of your file without changing its content. 
   The GST website will only accept "JSON" files if you re filling your "GSTR1" in offline mode. You may have 10,000 or more invoices to upload online, but this "JSON" file will reduce the size of your file and your supplies data will be uploaded successfully. You can also export "GSTR1" from tally and convert it to "JSON" file.
   Now let's know how to covert excel file  into "JSON" format. First thing you need to download "JSON" file converter which is not yet available on GST website. So click here on link  to download "JSON" file converter https://jb64.org and download it.  This installation file is in zip format, extract it and click on ExcelConveter.exe. (green icon)


  A dialog box will open, browse and select the excel file you want to convert. Destination will be auto selected based on your selection of file, if you want you can change to your desired destination.
    If your excel file is error free than another dialog box will get open and by auto string will be selected, click on next and you will get message in the same box that conversion started and conversion successfully completed.  If you get error, please regenerate the excel file and try again. 

     Now go to folder you had selected as the destination for converted file. you will see one file will white icon and with extension ".jb64" now you can upload this file online and file your return.  click on below link to watch video on how to covert excel into JSON format  https://youtu.be/l_iCfZwmfUg