I am a semi self-taught seamstress. I learned the basics of sewing from a few classes and now when I have an idea for a new project, I turn to the web for tips and tutorials. Here are a few of my favorites:
For a more advanced cushion project, try your hand at quilted throw pillows with this tutorial from the Purl Bee.
For a basic envelope cushion, check out this tutorial from Happy Housie.
Any time I am adding a zipper into a cushion, I refresh my memory with this informative illustrated tutorial from Design Sponge.
My favorite kind of zipper to use is the invisible zip, and this tutorial from See Kate Sew will help you sort out how to correctly place one.
The piped cushion is a classic, and you can diy it by following these steps from Make It and Mend It.