If you have an LDD (Lego Digital Designer) file, usually extension .LXF, you can use that file to order the needed parts. I will guide you how to use my favourite page BrickOwl.

1. Create an account on www.brickowl.com.

2. Go to the Wishlist page.

3. Click the create a new wishlist.

4. Fill in a name.

5. Navigate to the Import area.

6. Select the LXF-filetype from the drop down.

7. Select your LXF-file and click Upload.

8. Lego parts added to the wishlist.

9. Navigate to the newly created and successfully imported wishlist to view a list of all lego bricks.

New step is to buy the parts in your wishlist. Follow this guide How to buy a wishlist from BrickOwl