Well, the easiest way is to cut the slot, ideally with a pass or passes with a table saw (router bits have a tendency to chatter and drift, especially in hard wood) then sneak up on the thickness of your inlay strip with a drum or belt slander. Since most folks don't have those, a well tuned table saw will do fine to cut your thin strips. In addition, use tinted epoxy ( with India ink, ebony oil stain, I've used both with success) to glue in the strips, it will fill and hide any gaps...