First of all, FlashButtons (Copyright FlashButtons.com) are navbars Neopets allows you to put into your guild. Here is an example:

Step 1: Download a 30 day free trial of Macromedia Flash MX here.

Step 2: Go to FlashButtons.com and customize your Navbar. (There are plenty of instructions for that there.) When you come to the downloading part, save the file as a Flash MX file.

Step 3: You will need to upload the Flash file. There are many free servers who allow this. I know GeoCities allows Flash files.

Step 4: Put the following HTML into your guild, and be sure to change the address to where your file is.