Here's a brief description of what you should do.
You need to open a terminal window (or a console window). Since you can do commands like cp and pwd I suppose you know how to start a console window. (It's somewhere in the kde menus, I don't remember where).
When you have a console you type the command
It will prompt you for a password, this is the "root"-password which you chose when you installed linux. Without the root-password you won't get any further.
Now, once you've provided the right password you have a super-user console session. This means that you can make any changes you like to your system. (Like administrator in Windows). Now it's time to stop the X-server. The X-server is a program which enables graphics on your computer One way to stop the X-server is to change the run-level. When you do this you will be promptly logged out and all your applications will terminate without any warning. You change runlevel with the command
Your display will go black, with white letters on it. After a while you are prompted for a username (if not, try to press return). Your computer is now at runlevel 3, it's fully functional with network and all, but no graphics. It's now time to log in as root, i.e. type root in response to the username prompt and press return. When prompted for a password you use the root-password. You are now logged in as root.
Now you need to find the .run file you downloaded. If you did that as your ordinary login-user it's likely that the file is in your home-directory. Say your ordinary login name is john, you type the command
(note the tilde immediately in front of the username). Now type the command
to get a list of all files there ending in .run. You should see the downloaded file. Say it's called NVIDIA.run, now type the command
This will feed the .run-file to the command-interpreter, you will get a couple of questions (accept license, and the like) and the driver will be installed. If all goes well it's now time to edit your XF86Config file. That is, you need to tell the X-server to use nvidia's driver instead of the one included with the X-server. Your XF86Config file is located in /etc/X11, that is, you may start an editor and tell it to edit the file /etc/X11/XF86Config. If you're not familiar with any non-graphical editor you first need to start X again, try the command
you'll get back your graphics and may choose a suitable editor from a menu. Before you do this you should make a backup-copy of XF86Config, in case you messes it up. Now, in XF86Config there's a line saying "driver nv", you should change the driver name from "nv" to "nvidia". There might be a couple of more changes to do, but it's in the README file. Once you're satisfied with the XF86Config file, you save it and stop the X-server, you may do this by pushing the three buttons Ctrl-Alt-Backspace (not Delete, but Backspace). When you now again issue the command
your X-server should start with the nvidia driver. If it works, stop the X-server and issue the command
to get back to runlevel 5, i.e. graphics. You're now done.
If it does not work, there might be a problem with the XF86Config-file, but I can't tell you what until you've tried.