If you have a cable adjuster somewhere, either at the frame housing stops or at the lever, start by turning tehm all the way in so that you'll be able to use them to add tension later. Also bring the shifter to the position for the innermost chainring.
Now the trick is to pull out as much slack as possible before clamping the wire. Usually it's difficult to get enough slack out, so I cheat. Shift the FD outward by pushing directly on the arm, then stick a pencil into the mechanism so it can't return all the way. That'll give you some extra ability to get the slack out.
BTW- there are lots of good (free) tutorials on the internet, both video and text with pictures. Search, "adjusting front derailleur" or "installing front derailleur" to find them. Review a few until you have a good sense of how it's done, then go back and finish the job.
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