Here is how I slipcovered the ottoman. I sorta winged it, so don't trust me :)
1. Because I had a pattern on my fabric, I ironed my pining line by eye balling where the pattern ended. See all those little points lined up.
2. Once I did that, I checked for fit on top of the ottoman. The ironed line matches up to the edge of the ottoman.
3. I pinned my piping as shown below with the raw edge pointing down. I used my ironed line as my guide. See how my piping stitching is pinned right over that line.
4. I pinned all the way around. Clipping around the corners just as you do with a pillow.
5. This is how I joined the piping. Overlap and cover with excess fabric. The picture explains it a little better, I hope. At this point, stitch with your zipper foot right over you existing stitch line on the piping. I learned to hug it real close. The closer the better.