create sequence diagram based on flow of events

Question

I have a use case - Maintain Timecard, a brief description of it and the flow of events, including alternative flows. I have to create a sequence diagram based on the information given from description and flows. I used to work a little in Astah, but now I have to do this in Visual Paradigm.

I would like some help in creating this diagram, so it would be an example for further use cases.

This is what i did for now, but I'm not sure it's entirely correct:

Information about use case:

Maintain Timecard

Brief Description

Allows the Employee to update and submit timecard information. Hourly and salaried employees must submit weekly timecards recording all hours worked that week and which projects the hours are billed to. An Employee can only make changes to the timecard for the current pay period and before the timecard has been submitted.

Flow of Events

Basic Flow

Starts when the Employee wishes to enter hours worked into his current timecard.

1. The system retrieves and displays the current timecard for the Employee. If a timecard does not exist for the Employee for the current pay period, the system creates a new one. The start and end dates of the timecard are set by the system and cannot be changed by the Employee.
2. The system retrieves and displays the list of available charge numbers from the Project Management Database.
3. The Employee selects the appropriate charge numbers and enters the hours worked for any desired date (within the date range of the timecard).
4. Once the Employee has entered the information, the system saves the timecard.

Submit Timecard

1. At any time, the Employee may request that the system submit the timecard.
2. At that time, the system assigns the current date to the timecard as the submitted date and changes the status of the timecard to submitted. No changes are permitted to the timecard once it has been submitted.
3. The system validates the timecard by checking the number of hours worked against each charge number. The total number of hours worked against all charge numbers must not exceed any limit established for the Employee (for example, the Employee may not be allowed to work overtime).
4. The system retains the number of hours worked for each charge number in the timecard.
5. The system saves the timecard.
6. The system makes the timecard read-only, and no further changes are allowed once the timecard is submitted.

Alternative Flows

Invalid Number of Hours

If, in the Basic Flow, an invalid number of hours is entered for a single day (>24), or the number entered exceeds the maximum allowable for the Employee, the system will display an error message and prompt for a valid number of hours. The Employee must enter a valid number, or cancel the operation, in which case the use case ends.

Timecard Already Submitted

If, in the Basic Flow, the Employees current timecard has already been submitted, the system displays a read-only copy of the timecard and informs the Employee that the timecard has already been submitted, so no changes can be made to it. The Employee acknowledges the message and the use case ends.

Project Management Database Not Available

If, in the Basic Flow, the Project Management Database is not available, the system will display an error message stating that the list of available charge numbers is not available. The Employee acknowledges the error and may either choose to continue (without selectable charge numbers), or to cancel (any timecard changes are discarded and the use case ends). Note: Without selectable charge numbers, the Employee may change hours for a charge number already listed on the timecard, but he/she may not add hours for a charge number that is not already listed.

PS: I know I'm asking for a lot of time, but I would really like to understand how properly to do this, because if I start wrong, it will lead to nothing.