The first and most important step of all is to make sure that your still images and footage are top quality. This involves making sure you can correctly compose and expose to a professional standard - and then capture content that is commercially viable. Editing - i.e. reducing down to only the absolute best images or footage - is then the next crucial step.
Adding material to a stock library is easy, but you won’t necessarily achieve anything other than becoming disappointed if you neglect the above advice. Customers are usually demanding and simply won’t buy rubbish (unless it’s your mother doing the buying out of sympathy). It’s also important to establish a reputation for top quality work with customers if you want to consistently sell. Most potential customers will simply bypass your work if it contains anything mediocre or sub-standard, so make sure you only include the very best.
It’s probably worth adding that I know what I’m talking about through many years of professional experience. An unprofessional approach will fail. So, yet again repeating myself, if you fail to prepare you’re preparing to fail. Composition, exposure, commercial content, and edit to only include/upload top quality material.