I tear a 3/4" wide piece of 0.007" thick carbon laminate for use on the bottom sheeting.
The carbon should be placed approximately at the thickest section of the wing. For this wing it was approximately 7" from the trailing edge.
It was tacked in place with 3M77 adhesive.
The following site shows some 48" long pieces:
http://www.cstsales.com/carbon_laminates_48.html