The Design
Begin by deciding what subject you wish to re-create in Lego form. Whether you are looking to build a model of your house or a life-size replica of your dog, you'll need to gather as many reference pictures as possible for your use. Once you have pictures from every angle, it is time to translate this image into a blueprint for Lego construction. Because Legos tend to be more blocks than spheres, you should use graph paper to create the blueprint. Draw a simplified version of your subject on the graph paper using a pencil. Like your reference photos, you'll need to draw several angles of your subject, unless you are only translating a flat object. Once the object has been drawn, go back and fit each curve into the squares on the paper. For example, a round eye on your subject will be translated into your blueprint as several squares forming what is more closely resembling a hexagon. Remember that the more Legos you use and the bigger your design, the more detail it can give. Leave out any tiny details for now, as these will be added once you examine the building blocks available to you. Go back over your design and add the colors that each Lego brick needs to be.
Learning About Legos
Once your basic design has been created, it is time to learn about what Legos can do. Visit a Lego Marketplace to see images of every Lego available. You'll find clear blocks, or gears that can be used to create moving parts. For those who are re-creating a painting with Lego, you will find that there are countless colors to choose from. As you look at the blocks, begin to label your blueprint with the types of blocks needed. This can be as simple as choosing a long thin brick over a standard four-by-four brick to create a line of color, or as complicated as choosing the correct Lego windows to mimic the bay windows of your dream summer home. Once you have all your Lego pieces, build your creation according to your plans. While building, note on your designs any areas that don't work in practice or that can be done with a better combination of bricks. By the end of construction, your homemade Lego design will be complete along with your Lego creation.
Tips
If you are having trouble starting your design or figuring out the right combination of Legos, take a look at existing Lego models and instructions. Adapt any ideas you see to your own creations. For example, if you're looking to build a model of your town, look at the Lego City collection. For models of people and animals, consider how Lego people look with yellow heads and block-shaped bodies. By expanding on the same basic framework, you can design your own creation.