Step 1: Create a voxel design of your own! See How To: Make a Voxel Design.
Step 2: Click "My Stuff" on the header, then click "My Designs".
Step 3: Click on the design you wish to submit.
Step 4: Verify the name and description are what you would like them to appear as on the Marketplace.
Step 5: Click the "Submit to Marketplace" button under the design's image.
Step 5: Choose a suggested price in BuildBux and/or Qbits to sell your design for. Suggested prices may be subject to change during the approval process.
Step 6: Choose the design type. Use the legend provided for help determining the design type.
Step 7: Press the "Submit" button at the bottom of the page.
See also: Design Submission Guidelines.