Knowing how to replace the rubber seal at the back of the toilet isn’t hard but there is a trick to it tho. The rubber seal on the back of the toilet where the pipe connects from the toilet cistern to the pan is called pan cone rubber.
Pan cone rubbers come in two sizes:
The size of the rubber that fits on to the toilet are all the same size, just the part fits on the flush pipe is what varies.
In the video below I show you the easiest way, the way that I do it and found to be the easiest.
This video will help you swap out the rubber on the back of the toilet where it leaks water every time you flush the toilet.
If you have a choice between key seal and pan cone rubbers, take the pan cone as I find they last longer and less prone to leak.