A LEDES file usually requires four pieces of ID information for the overall invoice (not line item). These are Client ID, Law Firm Matter ID, Client Matter ID, and Law Firm ID (see the Official LEDES 98B Field Specification and LEDES 98B specification for more detailed information).
To set up these fields, go to Configure→ID Fields (or click the shortcut icon:).
Each of these fields can come from any of the QuickBooks fields (or from a unchanging or Fixed Value shown in the following this configuration menu, including any custom field you define yourself in an QuickBooks Invoice Template:
The following shows an example of a typical looking setup for both QB Desktop & Online (many of the fields are common to both with some differences in naming).
ID Field Settings for QuickBooks Desktop
ID Field Settings for QuickBooks Online
The Client ID is often a single name/word of the end client (this may be defined in the third party website you are using to process invoices and will likely need to match that).
If one or more of these fields is not required in your case (often times the Law Firm Matter ID isn't required, simply select Fixed Value and leave the box blank (or sometimes a value of N/A will suffice if it can't be blank but is arbitrary).
Law Firm ID is often the tax id of your company and can be entered as as a Fixed Value as shown above.
To use the Job (or Sub-Customer) position 1 & 2. You will need to have two values in QuickBooks separated by just a space or a slash character.
The PO & FOB fields are built into QB Desktop and are available to use without further setup in QB, but you may consider using a Custom QB Field (set up initially under a Customer's Additional Info tab, and then added to the appropriate Template) as these fields allow you to pre-populate data in an Invoice based on data already filled in at the Customer or Job (Sub-customer) level.