Measure the length of your row of trailing blackberries.
Dig a 2 foot deep hole at either end of the row. Add a layer of gravel to the bottom of each hole.
Place a 4-inch by 4-inch treated wood post that is 8 feet long into each hole and backfill soil around them so they are planted securely.
Measure 20 feet from one post and dig another 2 foot deep hole. Add gravel to the hole and place a 2-inch by 2-inch treated wood post that is 8 feet long into the hole. Fill around it securely with soil. Repeat this step along the length of your row of blackberries, placing a post every 20 feet.
Unravel 3 feet of 12-gauge wire from the spool. Do not cut the wire. Wrap the wire securely around one of the end posts at about 5 feet from the ground. Unravel and stretch the wire 20 feet to the next post. Wrap the wire around the post. Continue unraveling and stretching the wire around all of the posts until it ends at the other 4-inch by 4-inch post. Wrap the wire securely around the post and cut it from the spool.
Repeat Step 5, but place the wire 18 inches below the first wire on the post.