We'll turn off the water supply to the cistern, flush to empty it, disconnect the supply pipe, and unbolt the toilet from the floor. We'll disconnect the pan from the waste pipe and lift the toilet out. Old toilets are heavy and awkward, so it's a two-person job.
We'll scrape off the old mastic or wax seal around the waste pipe outlet and clean it up. If the old pan connector's stuck in the pipe, we'll chisel it out. We'll check the waste pipe's sound and not cracked.
We'll connect the new pan to the waste pipe using a pan connector (flexible or rigid depending on what fits). We'll position the toilet, mark the fixing holes, drill and plug the floor, and bolt it down. We won't over-tighten the bolts (that cracks the pan).
We'll bolt the cistern to the pan (if it's close-coupled), fit the flush mechanism inside, and connect the water supply. We'll adjust the ballvalve and flush valve so it fills and flushes properly. We'll seal around the base with silicone to stop leaks.
We'll flush the toilet a few times to check it fills, stops, flushes cleanly, and doesn't leak. We'll check for leaks at the pan connector, the water supply, and around the base. Once it's all sound, we'll take the old toilet away and clean up.