Nina, most replacement gaskets leak. Try fluid master if you can find one but most of this stuff from China or Mexico and the manufacturing is faulty. See my other comment.
There are other illustrations elsewhere on the web.. The problem will be the new gasket will leak slightly . I just replaced one twice, can't stop the leak. Can't buy OEM parts just the generic. I have three toilets, tried to replace the gasket in all of them. Flowmaster gasket worked nothing else does. That's one out of four, one toilet shut off, going to have to do something else with a flusher, can't shut off anymore toilets, don't to try anymore gaskets.
Unscrew the cap where the little hose fits then remove the valve entirely. Or prop it all up and slide the gasket under it and then around its holder. If you try to slip it down the flush valve as indicated you may distort or harm the gasket or valve.