Go to http://groups.yahoo.com/group/stamphappy
(clicking on the link above will open it in a new window,
which will let you work on it and follow this tutorial at the same time)

On the menu on the left, click on FILES

Once you are in the FILES section, Click on SALES (or whichever folder you need)

When the folder in question is open, click on CREATE FOLDER
(if you don't need to create a folder, go directly here)

Insert the appropriate information:

then click on CREATE FOLDER to activate the creation.

Then you will see a list with your folder in it, click on your name (or the name of your folder) to open the folder.

Then, click on ADD FILE:

You will get to a window like the one below. Click on BROWSE to locate the file on your computer. Add a description if you wish, or your email address so people can reach you easily. And click on UPLOAD FILE.

When you are finished, post to StampHappy letting people know you have items for sale, that they are in the sales folder:
http://groups.yahoo.com/group/stamphappy/files/%20Sales/

Or tell them you have added a file in such and such folder.