Steps

1. Go to the folder you installed GmailDefaultMaker in (probably C:\Program Files\GmailDefaultMaker\)
2. Edit the file called link.txt
3. Erase all contents
4. Fill in the compose message link as one line in this format:

Format of the link:

Let's say that your email-provider is www.superemailprovider.com
Find its compose message url/syntax, so for example www.superemailprovider.com/sendemail.php?to=recipient_email@example.com&subject=Hello&body=Goodbye.
Format the link, using %RECI%, %SUBJ%, %BODY as placeholders for the recipient address, subject, and body. For example: www.superemailprovider.com/sendemail.php?to=%RECI%&subject=%SUBJ%&body=%BODY%