Check over the settings and make any amends. Then scroll down to the bottom to upload files to your CustomGPTs Knowledge. You can upload up to 20 documents. Think of this section as what knowledge you want to 'train' your CustomGPT with. It could be insight, strategy documents, examples of great fundraising ideas, anything relevant to help it do its job better. If someone has created a similar CustomGPT, check with them if they have any useful information you can re-use for yours.