Making a coat can be difficult, but it's not impossible. In fact, I used this very website to make the layout that I made for advice! I googled all over for advice.
GENERAL TIPS
For one, it is very important to have a good version of photoshop. The Gimp works really well, but there are other programs out there if you look.
Second, make sure that your coat is creative, but matches all of Howrse's criteria.
Third, you have to have the passes to get the Black Pearl. If someone does it for you and you want some credit for the coat, it doesn't matter. The coat will be counted as theirs.
HOW TO MAKE THE HORSE
This is the part everyone has trouble with. To begin...
Draw an outline on or off the computer. Scan it in if it is off.
You need a program to use. I use the Gimp, and highly recommend it.
The next step puts your observing skills to the test.
Note where the muscles are and the light comes onto a horse.
With this in mind, color and blur the horse, using different sized brushes for the main and tail.
I hope this helps for you all!