Ok, if you have your files in the folder stories on the D drive, they would most likely be in D:\Stories so here's what you do
Get to the Command Prompt as it was shown above in the Start Menu
D: <press enter> - This changes you to the D drive
CD\stories <press enter> - to clarify CD stands for Change Directory so it was CD above, not C
*ASIDE*
The above step can be done in two seperate steps: CD\ just to get to the Root of the D drive in case you were in another sub-directory and then CD stories to get into the stories sub-directory. The single step CD\stories simply cuts down on the number of steps by getting the the root and the switching to the stories directory in one step.
Continuing:
REN *.htm *.txt - the REN command stands for Rename, this renames all the files from .htm to .txt as stated above. The asterisk (*) is a wildcard, that is it is used to list ALL files. Similarly if you wanted to find all files that started with BOB, you could type DIR BOB* and it will list all files in the current sub-directory starting with BOB. DIR is the Directory command that lists all the files in the current directory you're in. You can find help for each of these commands by typing them alone and adding a /? after them, like DIR /?
If you have any other questions feel free to ask!
BTW: for all these commands (DIR, REN, CD) it doesn't matter if they're in upper or lowercase, I just kept them in uppercase to make it clearer.