Transaction code : MDBT

click Create Variant button (specified a variant name and click continue).

Then fill in the following fields :- 

Scope of Planning         : Optional (refer to Note if you want to specify a plant grouping)
Plant                             : Your company Plant
Processing Key             : NETCH
Create PUrchase Req.  : 1
Schedule lines               : 1
Create MRP lists          : 1
Planning mode              : 1
Scheduling                    : 1
Planning date                 : default date

Click the Back button and you will be prompt Save Values.

Type in a Description and click the Save button.

After creating the variant, park your cursor at the variant and click the Schedule job button.

Fill in the Date and Time value.

Click Schedule periodically and you are prompt to choose the period.

Click the Create button and you are done.

To check, you can click the Show Scheduling button.

Note :

Define Scope of Planning for Total Planning in 'OM0E'.

1. Choose New entries.
2. Enter a key and a description for the scope of planning that you would like to define. Save your entries.
3. Select the scope of planning and choose Sequence of plants/MRP areas.
4. Choose new entries and enter the plants or the MRP areas in the sequence in which they are to be planned. 
    The counter determines the sequence.
5. Save your entries.

SAP PP Transaction Codes
Commonly Used Tcodes in PP Module Part 1
Commonly Used Tcodes in PP Module Part 2
Commonly Used Tcodes in PP Module Part 3

SAP PP Books
SAP Production Planning, Certification and Interview Questions Reference Books

SAP PP Tips
SAP PP Tips and Production Planning/Control

Best regards,
SAP Basis, ABAP Programming and Other IMG Stuff
http://www.erpgreat.com