The process to install a plastic fence post is really not much different than installing a wooden or metal post. You need to lay out the fence line and where the posts will go, dig the holes, and put the posts in place. The steps given below will help you install a straight fence line using plastic fence posts.
Step 1: Fence Layout
Before you can set the first plastic fence post, you should take the time to lay out the fence line. Drive stakes in the ground at the corners, and stretch a line of string between the stakes. Mark where the posts will go by spraying a dot of marking paint at 10-foot intervals. There are two methods of running a fence, you can follow the lay of the ground, or you can maintain a straight fence top. Following the ground means that the fence will appear to rise and fall with dips and mounds. A straight fence top will have posts and fencing running in a straight line. Dips and rises are accounted for by digging deeper or shallower post holes, and the bottom edge of the fencing will sometimes be very close to the ground and raised up quite a bit in others.
Step 2: Post Distribution
Lay out your plastic fence. It is easier to install the fence if you already have the posts distributed than it would be to make a trip for another post at each hole. Doing so also give you the opportunity to make sure that you have all of the posts you need, and the accessories that go with them, such as post caps.
Step 3: Dig Post Holes
A general rule of thumb for a fence post hole is that it should be about a third the length of the post. The holes should be at least 1 foot deep, regardless of the length of the post. Use post hole diggers, and chop the soil up finely as it is removed. Most of this soil will go back into the hole, and breaking up during the digging will make it easier to use later.
Step 4A: Post Foundations
Whether or not to use a foundation around the base of your posts depends on the load the post will bear. In general corner posts should be anchored with ready-mix concrete, and the other posts can be braced by filling half of the hole around them with fine gravel. Short decorative plastic fences do not usually need to have foundations or other support.
Step 4B: Fill the Holes
Finish filling the holes with the soil that came out of them. Use a short piece of 2x4 to pack the dirt firmly. If you have a carpenter's level available, check the plumb of the posts as you go. To do this, first place the level on the south side of the post and adjust it, and then place the level on the east side of the post.