First you need a model to show. Rubs, chips, breaks, flaws, and even dust will hurt in the show ring so select carefully. You would not want to show a dirty or un-groomed horse in the real show ring.

You then need to pick a breed for your model horse. Do your research if you are not familiar with horse breeds. The confirmation must fit the breed you are trying to represent. Watch out for the color of the model as well.

If a company makes a model Andalusian then paints it palomino, you shouldn't show it as an Andalusian because they do not come in palomino! 

Also be careful of markings on legs or face.  There are breeds that come in very specific colors for example-dark bay.  If you have a dark bay model that fits a specific breed but your model has a snip on the face when the breed standard does not allow for any such markings, you shouldn't show it as that breed.  It will not do good.

You can of course repaint a model to the color of the breed of choice.  It then needs to be shown in the Custom division, not OF.