I'm not sure what time of the year your Martins show up.Our Martin scouts just showed up yesterday. The rest usually follow within a week. There are lots of web sites. just google purple Martin and you can find the conservation website among others. They need a broad fly zone so, set up your pole far away from any trees. Also, it will help keep predators away from your pole so, the birds aren't sitting ducks. We have had good success with two tier metal houses with railings. Haven't tried gourds or other material. These are easy to clean. My husband is trying a new technique of opening only a few holes on the house until the Martins establish because the sparrowa LOVE to try to settle in as well.
He used to take the house down and pull out sparrow nest a few times before enough Martins got in the boxes. You should have a water source near by. We have a neighbor with a pond so, they go there I assume.