OK, this is just a guide, but here goes.
Depends on what you want out of the car.
The correct repair would be metal replacement, but you do need the tools and a little skill.
There is obviously rust underneath that is coming through. Can this area be gotten at to neutralize the rust with something like POR-15? if you do not at least neutralize the rust, the rot will continue.
If you are not looking for restoration quality or it does not have to be perfect, then you can do a nice job with Kitty Hair if you have some bodywork skills and the tools to do it. Here is a quick YouTube video that give a little info on it:
What I would do is to use a body "pick hammer" and knock the rust holes in slightly to sink them just a little below the surrounding metal creating a "pocket/dent." Then fill in with the Kitty Hair. Let dry and then sand smooth. The surrounding good sheetmetal will be your guide in using your sanding block to contour/smooth down the Kitty Hair. If there is a "pocket" or depression left in the filler after sanding, then apply another light coat to fill the pocket in. Might have to do this a time or two.
If you have to "cut" the filler a lot to get it smooth after your first application, first use a 60-80 grit paper. Then put a second light layer of filler to fill any low spots and sand with a 120 grit. Let the filler dry real well, and then I'd put one more skim coat and sand with a 180-220 grit to finish it off. Once done and it feels smooth, prime the area and let dry. Rushing the drying process is one thing that often makes for a bad job. If you don't allow the layers to dry properly, the filler or primer underneath is still drying and as the solvents eventually dry out, the filler or primer can shrink and when it has a top coat of paint, that shrinkage will affect the color paint and that's when you can see where the repair work was done - so be patient and let the work dry.
Once you have it primed, you can then put a thin layer of "spot putty/glazing putty" over the primer. This will fill any pinholes or low areas. Once dried, then I like to go over it with a wet sandpaper like 220 or even 320. You will sand most of it off as you sand it down to the primer layer - you should not be getting into the filler layer if you have sanded the filler layer smooth, but if you sand into it lightly, you can be OK, just don't go any deeper. Once done, primer over it and that should be it. If you don't like it , another layer of spot putty and repeat.
All this can be found on YouTube as they have many tutorials on doing body work. This one is pretty good on filler and sanding:
You can also find stuff online to read. Your local autobody/paint store that supplies paints and materials to your local body shops should also be versed in helping you out and getting you what you need. Bring in a few picks of your car for them to see and they should be able to recommend for you.