Select a location for your clothesline posts that is in an open area with plenty of exposure to sunlight and breezes to dry your clothes. If possible, plan to have your clothesline running in a north-south direction to maximize the warmth of the sun. Avoid placing it under power lines or tree branches, as birds will soil your clothes while they hang on the line.
Determine the locations for the two posts, and mark the spots on the ground. Measure and confirm that the distance between the two marked locations is the same as your planned length for the clothesline.
Dig holes that are 36 inches deep and approximately 12 inches in diameter. Use a hand tamper to compact the soil at the bottom of the holes.
Add 6 inches of finely crushed gravel to the holes. Prepare ready mix concrete in a bucket or wheel barrow according to the instructions on the bag.
Have someone help you, and set the post in the center of the hole, standing up straight. Add the wet concrete to the hole, working it in well with a shovel to eliminate any air pockets. Overfill the hole with cement so that you can form a mound around the post rather than having a depression, which will allow water from rain or snow to run off and away from the post.
Use a level placed on the vertical edges of the pole to plumb it. Build braces on at least three sides of the post by nailing one end of a 2 x 4 piece of lumber to the pole and allowing the other end to extend down to the ground at about a 45-degree angle. Set the other post in the same way.
Allow the concrete to cure for 24 hours, and then remove the braces.