The idea is that don't want people wandering around the Nether then trying to get back into the Overworld by making a brand new portal. A corresponding Overworld portal could end up being made in someone-else's construction, which would be annoying for them and embarrassing for you (and hard to remove).
For this reason, we say
- Start with making a portal somewhere in the Overworld
- If it makes a brand new portal in the Nether, great.
- If it ports to an existing portal in the Nether, great also, but that Nether portal may not take you back to your starting Overworld portal. It will probably take you back to another Overworld portal to which it is better mapped.
- The solution is to carefully work out the Nether coordinates for the corresponding Nether portal to map from your Overworld portal. As others have pointed out above that is done by converting X, Y, Z to X/8, Y, Z/8. Then you can make a Nether portal at those coordinates in the Nether.
- If you get it slightly wrong, the new Nether portal will map to another existing Overworld portal.
- If you get it right, the new Nether portal will map to your existing Overworld portal.
- If you need help, ask here and one of the more experienced players will place the Nether portal for you. You will need to tell them the coordinates of your Overworld portal.
So I repeat, the key to not step on people's toes is always to make the portal on the Overworld first.