24 hours (i.e., 24 game days) after the request was taken, the plane will be delivered to the broker. At that time, automatically, it will be offered for you to buy (look for it in the aircraft market, under incoming private offers). You have up to 3 (real time) days from then to buy the aircraft. When you do so, you pay (all at once) the remainder. It's not "each month" and you can't choose how much to pay.
If, after 3 days from delivery, you haven't bought the aircraft, the broker can remove the private offer. You would, of course, loose the 30%-35% already paid.