Here's a relatively simple technique for using Photoshop to create a repeat without requiring a textile design plugin. The steps are taken from an online column by Frederick Chipkin, the author of Adobe Photoshop for Textile Design by Frederick L Chipkin.
1. Open Photoshop, open the image you want to be the basis of your repeat, then modify the canvas to reflect the size of the fabric you want to print (18"x22" for example).
2. Using the rectangular Marquee tool select the area for your repeat.
3. In the Edit menu, drag down to choose "Define pattern." In the dialog box, name your pattern and click 'OK.'
4. Deselect the Marquee (Select menu ->Deselect).
5. Go to the Edit menu and drag down to 'Fill.' In the Fill dialog box, next to 'Use" choose "Pattern." You can now choose your selected image as a custom pattern. Click OK.
6. The area of your canvas around the original image should now be filled with your pattern.
Note that in the example below, my original image does not 'match up' on the right/left and top/bottom, so there is a tiling effect. But it still looks nifty.