I had to do this job not too long ago. Pulled entire unit out separating the top seat portion from the base. I support the response for plumber grease liberally applied to flange. Installed base portion with flange reaching through the middle to make sure it didn't bind up while applying my body weight to the top of the base, and then tightened it down to where it would not rock from side to side. Then installed new seal and top (seat) portion. Did connections after. For me, this was not ideal for ability to reach everything and convenience, but after having it all back together and checking and finding leaks a couple of times...it was the only way I could get it to stop. Leak free for some time, (knocking on wood right as I press send). Good luck!