I just tie it with the bottom of the foam strip along the shank toward the hook point and then use some dubbing tight against the front of the post to hold it upright. The portion of the post pointing back to the hook point is covered with thread and helps form a natural taper toward the thorax. I tie the post tall until I get the hackle wrapped and secure and then trim it back closer to the shaft to cut down wind resistence on the cast. Keeping it longer at first helps in wrapping the hackle, I think.
If your foam post is thick enough, you can cut it at the bottom so you can tie half down to the shank in front and half in the back and it will stand upright without using a dubbing support.