Find the desire. Ask yourself what specific thing your character wants or needs at this point in the story and how he plans on pursuing it. The paragraph should center around either the character's attempt to attain his desire or attempt to figure out how to attain his desire. If your story is about Ritchie's quest for love, maybe this paragraph could tell the story of Ritchie's desire to call Jenny tonight and ask her out.
Find the conflict. Conflict is the lifeblood of storytelling, and because a paragraph is a mini story, it needs a conflict. Ask yourself if there is another character who would benefit from the protagonist not getting what he wants and how she could go about thwarting him. Or maybe it will be circumstantial: Ritchie wants to ask Jenny out, but he's so nervous he can barely talk, or he's worried about violating his religious beliefs or while he's on the phone with Jenny, Velma knocks on his door.
Think of a resolution. By the end of the paragraph, the character will have a new desire, either because he solved the conflict and attained his desire or a complication arose, setting up his desire to solve this new problem in the next paragraph. Ritchie swallows, breathes and calls Jenny, thus setting up his struggle to remain calm in the next paragraph. Ritchie decides to remain true to his principles, but he still needs someone to love. Ritchie is so surprised to see Velma he hangs up on Jenny.
Picture the setting in your mind. Try to imagine as many details as possible to make a real world for the characters and to provide yourself with ideas -- maybe an image of a certain piece or arrangement of furniture or a weird picture on the wall will change the way you think of a scene. Regardless of his motivation, Ritchie will talk on the phone differently sitting in a wicker chair than lying on a plush couch.
Examine the context. Ask yourself where to put the description of this scene and what work it can do for your story -- is it just to let the reader know what a place looks like, or can it serve as a symbol of some type or as a metaphor for the plot or the character's mood?
Visualize the setting through the story's eyes. Now that you know what work you want the description to do, reconsider what the details of the room look like and which to include. Space is valuable, and no one wants to read a long, drawn-out description if it is not somehow relevant to the story. Ritchie's living room will look bright and inviting to him if he is excited about calling Jenny; if he is nervous, it might look shadowy and intimidating.
Use original descriptive details to make the paragraph interesting and further develop the plot now that you know what mood you want to convey. Before you write the scene, find the strongest verbs and adjectives to describe the scene. Instead of "There was a green chair and an oak table," if Ritchie is nervous, say, "The chair was the same shade of green as the tiles in his elementary school cafeteria, a color he knew well from many long hours of staring at them. The oak table next to it was the same shade and texture as one of their peanut butter cookies."