When you get to the other edge you need to get your thread in position to go back the other way. You can either go through the connection bead next door and come back through the bead above it, or you can thread your needle under the thread between the two connection beads and back through your original bead. Use which ever method works best for the position your bead is in. Depending on the shape of your rock, you may have to increase of decrease (or both) to fill the gap completely.
When you reach the top of the gap, you will need to connect the fill beads with the edge of the second piece. The following illustration depicts an ideal situation where the whole row reaches the top at the same time. If that were always the case, all you would have to do is zip up the edges. Rocks being what they are, this is not always the case.
As each vertical column reaches the top connect it to the bead above it either by zipping just that portion if possible, or by back-stitching the beads together as in square stitch. If your work buckles, it's likely that you have used more beads than necessary to fill the gap. Go back and take out any extra beads. Don't panic, this is all part of the process when it comes to covering irregular objects!