Hello all,
Below is the process to create Line Items in DPP (Double Click for Publishers),

To create a line item first you need to create an order if the order is already created then you don't need to do that, else every time whenever you have new client/company ad to publish on your page first you have to create an order for that:

1. click on new order tab, then give the name to your order (i.e: XYZ ltd 2014).
2. After creating new order or if order already exist select/click on the same and the Click New line item.
3. Enter all the necessary information asked about the line item.
4. (i.e- line item name.) Line item names should be unique, inventory size of the creative (width * height)

Now in setting section,
5. Select the start and end date for the line item
6. Provide the quantity/number of impressions (not available for network and sponsorship line items, here you have to set the goal in %(percent) )
7. Cost, as well as any discount you want to take from the line item's cost.
8. In the Add targeting section, select the inventory you want to target. You can target ad units, placements, or both, also you can forecast result and check inventory targeted for the line items you can either browse through your network's inventory or perform a search.
9. Click Save and upload creative.
10. After clicking on the save and upload creatives it will take you to next window where you have to upload the creative you can do it by simply dragging the ad image/ creative, and the filling the information asked in below fields such as providing link on creative etc and then save.

Hope this will help you for more details please visit the below link

https://support.google.com/dfp_premium/search?q=how+to+create+line+items