Probably the main skill needed to restuff a couch is patience, and maybe a little bit of cutting and sewing talent. The best case scenario is that your couch allows you to simply zip open the cushion covering and insert a new piece of fitted foam. This is not unusual at all when it comes to the seat cushions, in particular. Many upper cushions, the ones you lean back against, are actually sewn to the couch and might not be quite so easy to restuff, but even these should not present an extraordinary challenge.
Before you begin restuffing, consider what kind of foam padding you want to use. Some people prefer a firm seat and others prefer to sink deep into it. Luckily, there is a broad range of foam available for purchase, allowing you to locate almost any size or shape, though you might have to find a custom cut. Take your time exploring the possibilities because you probably don't want to spend every weekend tweaking your sofa cushions for a perfect fit.
Once you have your new cushions perfectly placed and aligned is not the time to realize that you have no thread to match the color of the couch material. Before you make that first cut, it will pay off in saved aggravation by doing your color coordination first. Even if you think that horrible shade of lime green is safely hidden away out of sight, somehow, somewhere, someone is going to see it. If this makes a difference to you, plan ahead and think about how you'll finish before you start.
If your couch has an expensive covering material, like leather, and it is going to require cutting open a seam, you might choose to hire a professional upholsterer for the job. This advice applies only if you're unsure about your cutting and sewing skills. If you've got the skills and confidence, go for it. Just consider first the worst case outcome that you accidentally slice a big old gash in that high-dollar material, and how badly it would -- or wouldn't -- ruin your day.