First you update your version in Tally and go to F11-fetures fll the GST related details. After you create your ledgers and fill the details with GST related. Now you are ready to GST billing....
In your invoice cgst / cgst tax should be displayed clearly in 50-50% if out of state sale then only IGST should be displayed , your tax invoice should be combination of alfanumric serial number. HSN code also need to be display on invoice.