When I tell people I start my paintings with a layer of fabric, it can be difficult to understand, since the fabric itself often isn't apparent in the end result. Here's how I prep all my panels (and why I go through gel medium at a somewhat alarming rate).
I saturate the back of the fabric with gel medium, then brush medium on the panel. Then I press the fabric down and smooth it thoroughly, making sure there are no wrinkles or bubbles. To complete the process, I brush more medium on the front of the fabric, essentially encasing the fabric with acrylic and making it one with the panel. It's a sticky process, but I like having the fabric surface and pattern to work with, and it provides a great foundation for the rest of the painting.