Once you bought a pattern, you should have received an email with your invoice. In that invoice you have a link to the PDF files you purchased. That email sometimes gets lost or gets filtered as spam by e-mail services, so always check your spam/junk mail folder just in case.
You can always get your patterns by logging into your account at elfinthread.com again, using the username (e-mail address) and the password you submitted at checkout. Click on “My Account” section on the upper menu, if you are not logged in it will ask you for your credentials. Once you’re in, among the vertical menu there is a “Downloads” option (see the arrow):
There you will find all the patterns you bought, with a link to download them. There is a limit to how many times you can download a pattern, so use it wisely!