Figure 1: This is a very common problem. The welt on this seat cushion has worn through and the seam has begun to come apart.
Figure 2: Carefully cut the seam open, cutting only the thread. Cut past the worn out leather until good is encountered.
Figure 3: Sometimes the welt is only sewn into the seam. Here the welt has been sewn to the boxing before the boxing was sewn to the facing. So an additional cut is needed, to remove the welt from the boxing. Cut the thread about one-inch past the point that the welt is ruined.
Figure 4: Remove the ruined section of the welt, inspect the condition of the leather, and remove the thread remnants. Most of the leather is OK, but there is one bad area that will have to be worked around.

Figure 5: To insure your new stitches are sewn in good leather chalk around the bad area. Keep about 3/8 inch away from it and smoothly taper the ends. Follow this line with your new stitches.
Figure 6: The thread in your needle needs to be as long as the area to be repaired plus 18 to 24 inches to insure enough thread to complete the job and bit to spare. White is used so it will show up in the figures. Normally it is a colour matching the leather.
Begin about an inch past where the thread had been cut, where the old stitching is still holding the seam together. From the inside of the seat cover push the needle through an existing thread hole in the facing. Pull until almost all the thread has come through leaving about a six-inch tail behind.
Figure 7: Using the existing hole in the welt that corresponds to the last hole used, push the needle through, and all of the thread except the six-inch tail.
Figure 8: From the outside of the seat cover, using the existing hole in the boxing that corresponds to the last hole used, push the needle through into the inside of the cover and all of the thread except the six-inch tail.

Figure 9: Using the six-inch tail left in Fig 6, tie a slipknot around the thread, and pull it tight. The knot will disappear inside the seat cover.
Figure 10: From the inside the seat cover push the needle through the next existing thread hole in the facing. Pull the thread tight.
Figure 11: Using the existing hole in the welt corresponding to the last hole used, push the needle through and pull the thread tight.
Figure 12: From outside of the seat cover, use the hole in the boxing corresponding to the last hole used, push the needle through that hole and the next existing hole. Pull the thread tight. An existing hole was missed out and second one over used. Either way is OK providing the corresponding hole in the welt and facing are subsequently used.
Now it becomes obvious that a curved needle is necessary as it allows you to put the needle through two holes in the leather at the same time.

Figure 13: Using the existing hole in the welt that corresponds to the last hole used, push the needle through and pull the thread tight.
Figure 14: From the outside of the seat cover, using the existing hole in the facing that corresponds to the last hole used, push the needle through that hole and the next existing hole in the facing. Pull the thread tight.
Figure 15: Position the thread over the welt. From outside of the seat cover, use the existing hole in the boxing corresponding to the last hole used, push the needle through and also the next hole in the boxing. Press the end of the welt down into the seat cover and pull the thread tight.
Figure 16: Again position the thread over the welt, from outside of the seat cover, use the existing hole in the facing that corresponds to the last hole used. Push the needle through that and the next existing hole in the facing. Press the end of the welt down into the seat cover and pull the thread tight.

Figure 17: From outside of the seat cover, use the existing hole in the boxing corresponding to the last hole used, push the needle through that and the next existing hole in the boxing. Pull the thread tight.
Figure 18: Continue this process up to the beginning of the chalk mark. As you can see, every other hole will be used
Figure19: Now, instead of using existing holes in the boxing new holes must be made on the chalk line that correspond to the existing holes in the facing.
Figure 20: To ensure proper alignment between the corresponding facing and boxing holes, pull the thread out from the previous hole and hold it at right angles to the seam. Make a new hole where the thread intersects the chalk line, but only partly push the needle through.

Figure 21: Again, using the thread as a guide, pull it tight and hold it at right angles to the seam in line with the next hole in the facing. Where the thread intersects the chalk line, push the needle into the leather to create a new hole.
Figure 22: Push the needle through the two new holes and pull the thread tight. Continue this process until the chalk line once more contacts the existing holes.
Figure 23: Notice how doing this has closed the seam and nicely finished the end of the welt.
Figure 24: Press the end of the welt down into the seat cover and stitch over it.