re: How to

Maybe with veneer?  Start with a white piece of wood.  Cut thin grooves to inlay some strips to delineate the individual white keys and glue some black pieces of veneer on top for the black keys.  The only saw I can think of that is thin enough to cut cut the delineating lines may be a flush trim saw?  You could also fill the the kerfs with a black epoxy or CA.  You could simply use a v-gouge to carve a very thin line to delineate each key  instead of filling the gap.   You could also use dremel router plunge tool and a v-bit to cut the lines.  

Another option may be to glue small pieces for each key together with a contrasting piece of thin veneer between them to delineate the individual keys. If you use a dark glue, that might be enough to make the keys stand out and the veneer between each key may not be necessary.