Hello there! The provided links didn't seem to contain specific instructions for tightening sofa springs, but I can certainly help you with that based on general furniture repair knowledge.
Fixing sagging sofa springs is a very doable DIY project that can bring new life to your couch. The first thing you'll need to do is flip the sofa over and carefully remove the thin fabric dust cover on the bottom, which is usually attached with staples. This will let you see what kind of springs you're working with.
Most modern sofas have serpentine springs, which look like long, S-shaped zig-zag wires running from the front of the sofa frame to the back. Often, the sag comes from the metal clips that hold these springs to the frame becoming loose or the springs themselves stretching over time. A great way to tighten these is to add support. You can run a strong, taut wire (like baling wire or special upholstery wire) across the springs from side-to-side. Attach the wire securely to the wooden frame, and then use smaller pieces of strong twine to tie each zig-zag spring firmly to the new support wire. Adding two or three of these support wires across the seating area can create a lot more tension and significantly reduce the sag.
If your sofa has older, individual coiled springs, the problem is usually that the twine tying them together has stretched or broken. Fixing this involves a more traditional upholstery technique of re-tying the springs with new, strong upholstery twine. This is a more complex job but is the proper way to restore the tension in a coiled spring system.
For a much quicker fix that doesn't require any tools, you can place a piece of thin plywood or a specially made sofa support board under the cushions. This doesn't actually fix the springs, but it provides a firm surface that counteracts the sag and can make the sofa feel much more supportive.
After your repair is complete, you can simply use a staple gun to reattach the dust cover to the bottom of the frame.
