Here's one way to make those seamless patterns. The first step is to determine the size of each tile.
Smaller tiles download faster for Web backgrounds, larger tiles allow more room to work and play.
We'll use a 50 pixel by 50 pixel sample, so we need to work on a canvas that's at least 100x100 pixels, with a transparent background. The first thing I do is make the grid visible and snap-to, using the appropriate commands under the View menu. What we're aiming for is a pattern that seems to have no distinct beginning or end. The way we'll do this is to have elements that run off the left edge re-appear on the right side of the tile. First we draw some things hanging beyond the left edge.