WIND and THREAD has some pointers for when and how to fly your private signal.

First, we recommend that if you choose to fly your private signal between 0800 and sunset, the signal should be hoisted after the ensign and struck before the ensign. Some reference sources invite the use of a private signal at any hour, though good sense indicates that a private signal flown at night should be illuminated.

Sailboats:
The private signal is a very elegant accent when flown at the masthead of your single-masted yacht in lieu of your yacht club burgee. Most sailors have a stern flagstaff used for the ensign; if you do not have one, consider installing something for the ensign. If your sailboat has a gaff-rigged main, we recommend you fly the ensign from the leech of the main sail, while flying your private signal from the masthead. If you have a schooner, a ketch, a yawl or a vessel with two or more masts, feel free to fly your private signal from the masthead of the aftermost mast.

Powerboats:
The best place to fly your private signal would be from the top of your aft mast on your two-masted motor-yacht. If you are not a Vanderbilt with a two-masted motor-yacht, keep reading. If you have a stern staff, we recommend you fly the ensign from that and install a bow staff. You can fly your private signal from the bow instead of your yacht club burgee. And if you are lucky enough to have a single mast, as well as a staff on the bow and stern, use the masthead for your private signal.