You have to charge GST on invoice amount.
For example, If you supplied goods for 100 Rs. then charge GST on 100.
If you received 100 Rs. including GST, then do the reverse formula.
100/(100+gst rate)*GST rate
For example if your GST rate is 5%, and you received 100 Rs., then bill amount should be 100/105%*5%
Rs. 95.23 will be bill amount and 4.77 be tax amount.
Round off for non decimal numbers.