There are two ways: The easiest: if the work you have done was pursuant to a saved Event or a Task, then all you have to do is tap “Send to Time & Expenses” and then fill in the number or hours and billing rate. (If you used the clock in the Event or Task screen then you don’t even have to fill out the time.)
If the work was not the result of an Event or Task, go to Time & Expenses, tap the “+” time button, and then fill out the date, amount of time, and hourly rate.