Illustrator Patterns have baffled me for years and I have always avoided them. But now I no longer have to be afraid of and frustrated with them. Here is a tutorial for making a simple fishscale pattern.
What's different about this method is the order you do the steps. Most pattern tutorials have you start with a square (clipping mask), and then tediously fit all your objects within the square. But with this method, you can just make a design that looks cool, and then get the measurements you need to make the correct sized square/clipping mask. Its much more designer-friendly.
1. Create an object.
2. Duplicate it a few times.
3. Group the objects.
4. On the group: Effect > Transform > Horizontal (write down the increments you used for the horizontal transform)
5. Make a second effect: Effect > Transform > Vertical (Write down the increments you used for the vertical transform.) "This will apply another instance of the effect." Click OK.
6. Make a no-fill/no-stroke rectangle using the measurements you wrote down from the horizontal and vertical transform.
7. Drag the rectangle to over your objects. Anywhere is fine. It doesn't really matter where. Just make sure it's not too close to the edges.
8. Send the rectangle to the back.
9. Select both the rectangle and the group of objects and drag the whole bunch into the swatches panel.
10. Drag out a new square and fill it with your new pattern swatch.
One of the coolest parts about this method is that because it uses live transform effects, you can edit and change the pattern design with a live preview, BEFORE making the Pattern Swatch. Simply double click to go into the group and they can edit the pattern live! Then when you are happy with the design, you can create the pattern swatch by Option+dragging whole bunch over your old pattern swatch to redefine it.
Just by changing the colors of some of the circles, and scooting them around a bit, I got a totally different pattern.
This pattern uses a square, but you can use a rectangle, too. Just be sure to use the measurements from the transform dialog box. (Negative transform numbers should be used just like positive numbers when calculating your rectangle size.)
If this peaks your interest in Illustrator patterns, here are a couple of other good resources I found: