I put on a "knocked down" finish on my walls and ceiling in my office. What you have looks just like what I did before I knocked down the peaks. I tried two different methods to apply the drywall compound. The first was to load up a damp sponge with the drywall compound, push it against the wall/ceiling and pull it straight away. Do this repeatedly on your patch area, twisting your wrist between each contact. This will leave random peaks. I used the compound straight from the can, not thinned.
The other method I used (which was much faster) was to buy a roller which goes on a regular paint roller, that is made for this kind of texture. I think they call it a texture roller. This would be overkill for your small patch though.
It's pretty easy to do, but I would practice on some scrap first until you get the technique down.
Good Luck. /forums/images/graemlins/smile.gif