You should use the custom form on their site, it will have a picture showing the different areas of the wig, so you can specify what density in what region (ie, if I recall, 1 and 2 are hairline and temples) and I think you want 100% in region 1 and 2 and 150% throughout. I don't think you want lighter; much easier to remove hair than add it back, lol. IF you have some clear pictures of the wig you have 0 if you like the density (I forget) maybe we could guide you better.
You don't want to say 'medium' or heavy' - spell it out for them!
The form also has a place for notes, and you can say "please place silk top 1/8 of an inch from hairline edge" there. Yes, write INCH! Spell it out. I like to use PLEASE and THANK YOU when adding notes.. just to be nice, you know?
The best way to work with them, IMHO, is send a filled out custom form, and then in your email when you send them the form, call attention to the details you are concerned about.