Hello there! Dealing with a sagging sofa is no fun, but the good news is that it's often a problem you can fix yourself. The context you provided didn't contain specific instructions, so I'll share some common and effective methods based on general repair knowledge.
First, you'll need to figure out what kind of springs you have and what the exact problem is. To do this, turn your sofa upside down. You will likely see a thin fabric, called a dust cover, stapled to the bottom of the frame. Carefully remove the staples using a flathead screwdriver or pliers so you can see the springs and the support structure.
The most common type of springs in modern sofas are zigzag or "S" shaped springs that run from the front to the back of the frame. Often, the sagging is caused by the clips holding these springs to the wooden frame breaking or coming loose. If a clip is just loose, you may be able to secure it with pliers or by adding a new screw. If a clip is broken, you can buy replacements online or at an upholstery supply shop.
Sometimes, the springs themselves have stretched out over time and lost their tension. A quick way to add support is to run strong twine or wire perpendicular to the springs, weaving it over and under each one. Pull it as tight as you can and staple it securely to the frame on both ends. This creates a tighter web that provides more support. For a more permanent fix, you would need to replace the stretched-out springs entirely.
Another simple, no-tools-required solution is to place a firm piece of plywood or a specialty sofa support board cut to size directly under the cushions. This doesn't fix the springs, but it provides a solid base that can significantly reduce the sag and make the sofa much more comfortable to sit on.
Once you've made your repairs, you can reattach the dust cover to the bottom of the sofa using a staple gun to give it a clean, finished look. With a little investigation and effort, you can often bring new life to a sagging sofa
