You need to first create a promotion as follows: Tools | Promotion | New Promotion: named FOC with discount set to 100%. Then do as follows.
1. Go to Financials | New Bill | Select Patient. A New Bill window will open.
2. Select +Add Items to add services or products.
3. Click on the + button beside Discount and a Promotions flyout will open.
4. Select Free of Cost(FOC) from the list and Save.
If you do not have a pre-made discount promotion called Free Of Cost(FOC), simply enter the Discount against each item in the Bill as 100%.