Fix a Mansfield 160 "hard flushing" toilet?

I've looked at the other SAME problems, and am quite sure that I've found the problem....replacing the flush valve seal. The price of the seal is about $3 and the whole flush valve it about $8. 'Appears as though for about $5 I can ASSURE that it'll work all right, with all new parts..and not much disassembly/reassembly??

Also, is there a "how to" video/printout w/pics, or anything to give me a heads up on HOW to do it? I'm SURE that "some" of the attachements of the parts to one another is by threads, and some not??? HELP!