The inlay he does below is a fairly complicated one I would recommend practicing on some scrap wood before attempting something this complex.
Using inlays to decorate your projects is a great way to create a distinctive feel and a product that is sure to impress. Getting good at inlays really is just a matter of practice, keep at it and soon you will have added another skill to your woodworking skill set.
If I have small gaps, I fill them with a solvent-based putty before finishing, alternatively I use a soft wax-based putty afterwards. I have found that cover up knots in the wood with a simple inlay can be very useful.