How can I add my 1 of 1 NFTs?

1. Use the traits tab to build an NFT and save it as a Custom NFT.
2. Create a folder called @custom in your traits folder and add files (images, gifs and videos) to it, then use the “Add asset” button above under the Custom NFTs tab next to Traits 

There's a step-by-step guide on this process here as well.