Putting a volumetric flask on balance. Add the butanol into flask, read the balance reading, and top up the water to the volume line on volumetric flask.
Mutual solubility for this system can be examined after A.F.M. Barton (Ed.), IUPAC ― Solubility Data Series, Vol. 15 ― "Alcohols with water", Pergamon Press, 1984.
While considering n-butanol recovery or drying ― at this forum: https://www.researchgate.net/post/How_to_evaporate_N-butanol_fractions