Architectural descriptions aren't easy. Unlike architectural photography and drafting, they don't require special equipment and unfamiliar skills, but they do require an ability to recognize and interpret building materials, structural systems, decorative features, the relationships among all these elements, as well as evidence of additions or alterations. You also need to know the names for everything. Mastering the necessary material may seem like on overwhelming task, but you can take heart in the assurance that a certain amount of concerted work with vocabularies and diagrams and standing buildings will pay off pretty fast. But identifying and understanding what you see isn't quite enough.
Buildings are very complex artifacts.
The observations it's possible to make about even a small and plain structure are bewilderingly numerous. Next to a good working knowledge of architectural components and terms, the most important thing you need is an effective system for organizing your description. What follows is a good format, adapted from the one recommended by the Historic American Buildings Survey. After a basic identification and brief summary statement, it treats, in sequence, the building's site, exterior, and interior, It proceeds from the general to the particular, from the structural toward the decorative, from the outside in, and from the ground up. You get it by the scruff of the neck, so to speak.