When making a birdhouse where exactly should the hole or entrance be?

A door to clean out would be really nice, since some birds will go back or other birds will find it. You should probably allow 2 to 3 inches from the bottom to the entrance hole.

I find the center and go about 1" below. I either make the bottom or one roof panel removable for cleanup. Depends on the style of birdhouse.

You do need a door to clean them in the fall. A simple flip up door on the front with a simple latch will do.More important is the size of hole you need for the bird you are trying to attract. Most bird house holes are centered. Look at the ones in a store like Lowes and measure if you want.

You do need to clean them out. Some birds will not return and what ends up happening most times is squirrels end up making it home. The hole placement is not as important as the dowel, or some place for the bird to land before it goes into the house.

Each birdhouse should be built with the specific bird in mind. A bluebird's box is different in size, shape and hole placement when compared to, say, a house wren (example). I strongly urge you to Google the specific birds you want to attract, and build for them based on nest boxes most sites will provide. You'll want to place a catch on one of the walls to allow you to clean it. This is important as birds may use the nest box up to three times per nesting season as well as for roosting at night, in storms and in winter. Birds build different types of nests, and they will build it as high (or low) as they need. If they have to build on an old nest, the nest would be higher, thus encouraging predators. (Also, some nests are just gross with bugs, unhatched eggs, etc.) Most birds do not use perches. Depending on the bird, they may need the entry wall to be scuffed up or not. Placement of the nesting box is also important. Thank you for providing for the birds; it's a wonderful hobby.

Different birds need different size holes.....If you google birdhouse plans, you will get an idea of the many different requirements to attract various birds in your area. Some birds need a smaller hole to prevent predators from coming in, so need larger, etc. Also the height in important, depending on the bird species.