What is the appropriate procedure for determining the quantum numbers of an electron?
1s2 2s2 2p6 3s2 3p6 4s1 is the electronic configuration of the 19th electron of chromium.

since the last shell is 4, therefore n=4,

l=0(as there is only one orbital in the s sub shell) ,

m=0 (since we're still in the s sub shell only) 

and s=+1/2 or s=-1/2(arbitrarily assigned values)

note:+1/2 or -1/2 are used to distinguish between electrons as no two electrons can have the same electronic configuration, but here the question isn't that specific, so we need not bother.