Imagine
First, decide what you would like to build. You can create a LEGO set about anything you'd like. Examples of LEGO sets are a house, a store or a building. Or they can be cars, flying saucers or scenes from your favorite TV show. Come up with your own idea and spread out your LEGO pieces. Plan out what you're going to build and which pieces you might want to use.
Build and Record
Begin to build your LEGO set. Each time you add a piece or take one away, write it down. Use the same descriptions that you find in instructions manuals for prepackaged LEGO sets. Explain the colors of the pieces you are using as well as the size of pieces. At each stage of the process, take a photograph with a digital camera. This will help you explain what the process looks like.
Preserve
When you think your homemade LEGO set is complete, add details that you would normally find on a store-bought LEGO set. These might include headlights, windshield wipers or flower boxes for home or storefronts. Once you have the set looking exactly as you would like it to look, take one last photograph to record it. Print out each of the directions you have written down. Cut them apart and glue each direction to a different piece of paper. Print out the photos and glue them onto the paper next to each direction. Staple the papers together so you can build your LEGO set again.