Go to a multi-enter contest on FanDuel, choose to enter a new lineup, and then down toward the bottom (near the submit button) there's an option to upload via csv. Click on that and you'll get a link to a csv template with instructions.
Basically you need to list players in a format of one lineup per row with the top row the positional headings (QB, RB, RB, WR, WR, WR, TE, K, D) as in the template. In each subsequent row, list players by their IDs as provided in the template. (VLOOKUP is helpful for this.)