To create your own greasemonkey script, simply install greasemonkey, click on the icon and select New User Script. Then you will be prompted for some info about the script.
Name: what ever you want
Namespace: some unique identifier. A domain name you own is a good option
Description: What ever you want
After you enter this information, it will prompt you to select your editor and you can start writing your script. After you save, your script will be available in your browser when you refresh the page.
Finally, if you wish to copy your user script to another computer, simply save the script as a .user.js file and then drop and drag it into Firefox. Firefox will then prompt you to install the script and you can use it on the new computer.