a couple of things you need to sort first:

first you need to know what wood it is, as certain woods will take certain finishes, stains, etc without looks strange and others will soak up stains and finishes and polishes making them look blotchy

then you need to knon what finish has been put on it already, if there is any, as, again, different stains will react differently to other chemicals if you apply them over the top of one another

from what i've seen on new yankee workshop, darkening will require and oil or a wax coat on the main as opposed to a varnish or a finish, and the more coats you add, the deeper the colur and lustre becomes

and there is also the method of application as well, brush, cloth and foam brushing all have different methods of changing the hues, and there are some which you have to apply and then leave for a while before wiping off the excces and then leaving it to dry before giving it another coat and so on and so forth, again, this depends on the type of wood you have they all have different soak times

it also requires shed loads of elbow grease as norm has sometimes put on 3 or 4 coats of finish before applying a further several coats of polyurethane over the top to seal it all in and give it a shine as well

all them shows (17 series and counting) and finaly something i can put to use