Instructions
Decide on your creation. If it is a real structure, take a picture and have that available. Making a sketch from different angles is also a good idea.
Download LEGO Digital Designer (LDD) software from LEGO. This software is available for free from LEGO for Windows XP, Windows Vista or Windows 7 and Mac OS X 1.4 or higher. You will need 100 MB hard drive space available. Once you have downloaded, you will be asked to select your language and then the wizard will walk you through installing the program.
Choose one of either a starter model, free build or a recent model. If this is your first time using the program, there will be no recent models to choose from. The screen will show a blank LEGO plate in the center, categories of LEGO pieces on the left and other choices and help buttons on the right.
Click on a brick category to expand your choices. You can choose from almost every color, shape and size of brick available.
Drag bricks to the brick plate. If you are starting from scratch, you will need to completely build your set. If using a starter model or template, you can modify or add to the existing bricks on the plate. Make sure your bricks are connected. You may need to change the view with the right, left, up and down arrows on the screen to see bricks from all angles. You can save or print at any time.
Change from build to view mode when finished. Click the view button, which looks like two bricks stacked in the upper right of the screen (also F6). This will show your creation in 3D and allow you to look at it from different angles. Return to build mode if you need to make changes.
Check the price. Click the red brick with the price tag on it at the top right (Ctrl-B) any time during the design process so see how much the set will cost. If your design is too expensive, consider redesigning or scaling down your creation. In general, the price will be the cost of all pieces plus a service fee (approximately $10).
Upload your design to LEGO. Click the icon in the upper right with the globe and red brick (Shft-Ctrl-B) to upload to LEGO.com. You will be prompted to create a building guide and a custom box for your creation.
Complete your purchase. After uploading, you will be automatically directed to the LEGO.com shop with your design showing. Price and number of pieces will show, as they do for all LEGO sets. You can purchase a limited number of sets immediately.