The first thing you'll need to do is to remove as much covering as you need to so that you can see all of the damage. The diagrams show no covering at all just for the sake of clarity. You don't need to remove it all, but make sure to remove enough to give yourself room to work.

Now, inspect the damaged area thoroughly. Pay particular attention to the spars and make sure there are no breaks or cracks. If there are, you will need to glue them, or even apply some plywood doublers depending on how severe the damage is. Fortunately, in most of the repairs of this type that I have seen, the spars survive untouched, or with only a few minor dings or scratches which can be easily ignored. If they look damaged and you're unsure as to the severity, have a more experienced modeler check it out for you.

Once you're ready to begin, the next thing you'll want to do is to clean up the broken ends of the leading edge by cutting and sanding them to a clean end. You don't need to do the ribs, with the exception of removing any splinters that might interfere.

Get a piece of balsa that is the same thickness of the ribs (on 40 - 60 size planes this is usually 3/32"). Cut it with the grain horizontal so it fits between the spars and a good section of the leading edge.

Lay the balsa against a good rib-front and use a pen to trace the shape of the rib to the balsa.

Make a copy of this new rib-front for each one that needs replacing. The easy way to do this is to glue them all together with only a tiny drop of glue and shape them all at the same time. Then you can break them apart when you are finished. Glue one copy along side each broken rib.

Next, cut a block of balsa to fit between the ends of the leading edge. It would be a good idea to add some small scraps of plywood or hard balsa behind the seams for strength.