First of all you will have to find the right place where you want to build a seawall. Measure the area and check where the highest tide of water hits. This checking may take four to five weeks as high tides are associated with the phases of the moon. Make sure to accurately note the tides to get the proper measurements.
Now have another look at the measurements, length and width, of the wall and calculate the number of poles or posts. The most followed pattern is to fix a pole after every 8 feet. However, you can do the adjustments keeping in mind the nature of area and strengths of the tide.
After doing all the calculations, you should dig holes to install the metal poles. Again, the depth of holes depends on the nature of the soil. If it is hard enough then digging for two to three feet will be sufficient. Dig a hole after every 8 feet by the side of wall.
Next step will be to install metal poles. You should make sure that the pole is at least two feet in the hole. Remember, if the seawall is eight feet high then it is better to put four feet of pole in the hole.
Now you will have to fill the hole with concrete so that it holds the pole firmly. The concrete will take at least one day to dry so do not touch the base until it is ready for the next step.
Now you are all set to install wooden boards between the metal posts. Be careful while cutting the boards as any wrong measurement can turn a useful board into waste.
Drill holes in the poles and then fix the boards at each end of the poles. For this, you will have to seek help from somebody. As you cannot hold and bolt boards at the same time.