This method of creating butterfly or bowtie inlays uses a Slab Stitcher kit. This kit provides a simple template to create perfect inlays. They also sell templates to create other inlay shapes as well!
– Slab Stitcher Kit - Router - Correct router sub base to accept the bushing that comes with the kit - chisel - wood glue - sander
Each kit requires a sub base for your router that will accept the guide bushing that comes with the kit. If you're having trouble finding the specific sub base you need for your router, check out their website. They sell custom sub bases for common routers.
Place two Slab Stitcher templates on top of each other and set the depth of your router bit to the depth of both of those inserts.
Determine where you want the inlay to be and clamp the template to the proper location.
Plunge the router bit into the piece. Work clockwise to route the edges, then left to right. Continue until it is all removed.
Put wood glue on the bowtie and insert it into what you just routed out. Sand until it is flush.