To apply tax of item during voucher entry, You can follow the below steps:
Step 1: Go to transaction section then click on required voucher like sales, purchase etc.
Step 2: Click on add tab and mention the voucher number, date, party name, sales or purchase type, item details along with price and quantity etc.
Step 3: After specify all details, Press F4 shortcut key to apply tax at bill sundry in case of multi rate or specific tax rate sales or purchase type or apply tax at item level by press enter key on amount at item level in case of tax inclusive and item wise sales or purchase type.
Step 4: Next, save the invoice.