Proportion the costume to look good on you ?
L has a really simple costume but in the off chance that people haven't heard of Death Note it's always a good idea to post a reference picture or two.
Just buying pants intended to be worn by a man might solve the issue. Girl pants will almost certainly look very girly and accentuate anything you have. You don't want super tight pants, either.
How much you get "into" it is up to you and depends on what you want to accomplish. Do you care if people guess you're "really" female ? Do you want to 'pass' as a model XY citizen ? What do YOU want to do ? There is no wrong answer.