how do i order bases
 

i have a vintage 1950's custom rifle the maker of which i don't know. it's an FN receiver mauser action and a gartman barrel. the receiver is drilled and tapped with 1 screw in the rear and 2 on the front . what i need to know is how to order bases. do i have to give measurements? whats the 1st step? thanks for any help
don:confused: