Once you find a suitable sgf file, you must upload it to your OGS library (found on the sidebar).
Before you upload it, open the sgf with a text editor and delete any identifying information. This is a guessing game after all!
If you see a “server 500” error upon uploading the sgf to your OGS library, that means OGS is having trouble reading the file. The most common causes are usually blank fields that OGS expects to contain data. For example, OGS doesn’t like a blank komi field: “KM[]”. You need to delete it completely.