Bricklink uses either XML or a Lego Digital Designer file to mass upload a parts list; details on the correct formatting for XML can be found here. To upload a part lists to Bricklink, go to Want > Upload, and either drag the LXF file onto the page, or paste in the contents of the XML. Once that’s done, choose a wanted list to upload to and select Verify Items.
This step is for confirming quantities, colours, and parts are correct. Select the Exclude checkbox to not add an item. When everything is correct, clicking upload file will upload the parts to Bricklink.
Next: Now you’ve uploaded a parts list, check out how to find the best store to buy from.