- Choose a thread in the right colour (same as existing thread). Make sure the thread is strong enough to endure some force during the sewing process (and afterwards as well).
- Use the holes that are already in the leather. It gives you the same stitch size, and since you don't have to create new holes, it's easier for you as well.
- Make sure the stitches you use, resemble the stitches that have been used before. It looks like at one side, the edges of the leather are folded inwards, while at the other side, the leather looks like as if it's foldeld outwards. So, judge by similar parts (I'm sure the couch has another arm) to see what stitches you need.
Good luck, and I hope you can enjoy your couch a long time after the mending.
I would definitely heed the advice that Ji Ugug answer lays out. I would like to offer a technique that can help hide the stitches.
Start with thread that is way longer then it needs to be if you were just stitching this when you were not trying to hide it. What you would do is use the existing holes (assuming they are not damaged) and feed the thread through without pulling it taut. I would also go several stitches before and after the seam rip. Pull the end of the thread through somewhere it cannot be seen (under or beside the seat preferably, or near the back cushion). In the picture the seam seems large enough you could push your hand through a little.
Then, pull the thread tight (starting from the far end away from the needle) like a shoe lace. Pull the stitches tight as you go. Once you get to the end you can then try and tie off away from the repaired seem. Careful where you tie off as you can pull too hard and create a dimple in the arm.
Is this a bonded leather couch by chance? If yes you might have a harder time threading it as the holes will fail easier.