You will need:
Scissors or a box cutter
Hot Glue gun
Cotton batting or other stuffing
Hair clips or headband
Start by drawing a basic template of the ear you want. It could be bigger or smaller than the one I used here, just keep in mind that the actual ear will be slightly larger than your template. Also since its going to executed in fake fur and felt, any tiny details, like an extra pointy top, might get lost in the shuffle.
Use the template to cut out the inner ear out of felt-I used pink for the inner ear, you could use another color depending on your color scheme.
Now use your inner ear as a template for the outer part, the part in fake fur should be bigger on all sides than the inner part. I didn't get too picky, just cut about a half inch wider around the whole thing. Cutting really thick fake fur can be a bit tricky. I combed over the fur part away from the cut to avoid chopping off fluff. It might've been easier with a box cutter to cut the backing without losing fluff.
Once you have two inner and two outer parts glue the inner part to the back of the fake fur, the side that's showing will be the inner ear. This mattered on mine because I used a sparkly felt.
At the very top you'll pinch the excess fabric together, to get that little bit of bobcat ear tuft. You can then glue that pinched part so it doesn't show.
Once they're all glued down, add a little stuffing-I used some cotton batting, you could also just use cotton balls. You won't need a whole lot, cat ears aren't super thick but they also aren't completely flat. Add a little and spread it evenly. When you like the shape, you can fold up the last bit of fluff on the bottom and glue that to complete the ears.
The one on the right has some stuffing, the one on the left doesn't. I think you can see the difference.
I wanted to have clip-on ears, so I hot glued alligator clips to the bottoms. Unfortunately, because of the size I'd made these (4.5-5in) they just didn't sit right when clipped on. :(
Ah well, you live you learn. :) I ran to the nearest drugstore and found a decent sized headband; neither the thinnest nor the thickest headband, this one was probably a half inch thick. I hot glued my ears down where I wanted them and used a little extra fluff to disguise the band between em. Et voila!
They are still a smidge large for cat ears, but I like em! :)