If you ask anyone they will tell you that installing a toilet is not rocket science and for the most part they are correct.
For a new installation there is not very much that can go wrong provided the rough-in for the fixture was done to the proper specifications, however an installation of a new toilet for a replacement is a different story.
First of all when the old toilet is removed you need to have a close inspection at the wood underneath to determine whether or not there has been a slow leak from the previous fixture.
This would show up as a dark stained colour around the old flange and could possibly soften the wood to the point that a secure installation for the new one could be a problem.
In this case it would be recommended to change a small section of the floor with new wood and such a procedure could be achieved with very little damage.
The next inspection should be the actual toilet flange itself... and older plumbing system will have a cast iron flange with lead caulked around it.
The older flange has only two small slits in the side to accept the bolts and they aren't very forgiving. I myself would lock the bolts in place on the flange with a set of washers and nuts before setting the new toilet.
There is also a possibility that over the years, renovations have been done in the bathroom where a new layer of plywood and ceramic tiles have been installed, which would make the actual toilet flange sit lower in the floor than normal.
The first problem now is whether or not the closet bolts will be long enough to properly secure the fixture.
These bolts now have to extend from the old flange up through the new floor and tile and then through the porcelain of the bowl with enough thread to successfully install both the flat washer, the cap washer and the nut.
Even if you purchase long bolts, in some cases it isn't quite enough.
In a case like this there are repair flanges you can purchase which are around 3/8'' thick , made of plastic which can be screwed down on top of the original flange ( provided the original is plastic ) and raise the height of the lock down slots thereby bringing the toilet bowl closer to the level of the floor allowing the installer to use only one seal or gasket, be it sponge rubber (come in a variation of thickness ) or a wax seal.
Our preference on a wood floor is sponge rubber since the wood floor may still have some minor movement with the house and a wax seal once set is not going to change.
On a concrete floor they are great! In a two story house you want to be certain that this toilet is not going to be slowly leaking when you leave.
When you tighten down the bowl you can't be so loose that it will move when sat upon but on the other hand it can't be so tight that the porcelain of the bowl will crack over stress.
Over time you can learn a nice snug torque that falls somewhere in between and will stay in position forever.