Assigning fee to students is very easy, you just need to understand it. First off all
- Go to Cash In => Fee Invoices => New Invoice
- Fill the values
- Click Add button (As shown in the picture)

If the Add button is disabled
Go to Cash In => Fee Invoices => Invoice Status
Click Close This Invoice button

After generating the fee invoice
Go to Cash In => Fee Assignment => Assign monthly fee
Click Assign Monthly Fee button

For more help you can see this video