Within Natural HR there are three ways in which a timeoff in lieu allowance can be accrued by an employee.   

1) Timesheets - if you use timesheets and have enabled "Timesheets overtime auto accrue time off in lieu" in timeoff settings (https://www.naturalhr.net/hr/company/timeoff-settings) then, each time an employee submits a timesheet, any hours worked more or less than their contracted hours will be banked as time off in lieu. When the timesheet is approved the timeoff in lieu will also automatically get approved.

2) Adding manually by HR/admin - HR and admin users can manually add timeoff in lieu allowances for employees under HR/Timeoff management/Timeoff in lieu accrued.

3) Requests by employee - the employee can request the addition of timeoff in lieu allowance via 'Self Service' by clicking 'Timeoff in lieu allowance' in their main timeoff screen. This request will get routed to their manager for approval in the same way any request does.

For any of the above, and especially method three, you should note this simply adds the allowance to the employee giving them to ability to request timeoff in lieu and does not actually request any timeoff. 

Should an employee need to request the actual time off which they have banked then this should be done in the same as any timeoff request but, in this case, they would be drawing down against their banked hours.

Before trying to use any of these please ensure you have enabled Timeoff in lieu in Timeoff settings and also allocated a timeoff type for time off in lieu by setting timeoff in lieu to Yes in the time off type settings.