This may also apply to other versions of Corel Paint Shop as well, but Paint Shop Pro X2 is the version I use to make seamlessly tiling patterns. To see if your version of Paint Shop has the seamless patterns feature, look for it in the menu under "Effects/Image Effects/Seamless Tiling." If you don't have Paint Shop yet but want to get it, make sure the version you're looking at has the seamless tiling feature before you buy.
First off, not every image is going to work as a seamless pattern. Some will leave very noticeable lines along the borders if you attempt to make them seamless. You're going to have to find one that will work nicely, and sometimes this takes trial and error.
Just a note: I'm using a perfectly square pattern in this tutorial because it's a little easier for me. A seamless pattern used in creating custom content for computer games like The Sims doesn't have to be perfectly square. A rectangle would work, too.
Click on the pictures to make them bigger.
Making a Pattern Seamless
The pattern I chose is actually a small, cropped section of a photograph of sequined fabric. The best photos to use are ones that don't have shadows in them. If the lighting in a photo is noticeably brighter on one side of the photo than the other, don't use it, because it won't tile correctly. The best images to use have even lighting, and a general sameness and symmetry on every edge.
Bring up your pattern in Paint Shop. Click on "Effects/Image Effects/Seamless Tiling." It will bring up a window with a "Before" and "After" picture. Keep the setting on their defaults, which will be Edge Tiling and Bidirectional. You can play with the Direction settings, but you definitely want the Tiling Method to be set on "Edge."
Click OK. You'll see your pattern suddenly shift in front of you. You may notice a little blurring at the edges, but this is expected. It's now seamless! It's that easy. Except that you'll have to check it first before you can use it for a modding project, because some patterns will look terrible when they're turned seamless this way, and you don't want to discover it's terrible only after you try to apply it to custom Sims clothing.
Testing and Reducing the Size
To test it, go to "File/New" and create a new image that is exactly twice the dimensions of your seamless pattern. I generally make it easy on myself and start with a 200x200 pixel pattern, then make the new image 400x400 to test my new seamless pattern.
Right-click on your seamless pattern and click on "Copy." Then click on the new, blank image and click on "Paste as New Layer." Do this four times to put four versions of the pattern on the blank image. Each layer will appear on top of the other one, so you'll have to move them in order to see all four.
Find the "Move" tool in the toolbox. It looks like four arrows going in four different directions. Use this to move one square of your pattern to one upper corner, getting it lined up with the edges of the blank image exactly. The "Pan" tool (the hand, right above the "Move" tool) is used to grab the image, or to click on various open images in Paint Shop, but the "Move" tool is used to move layers within one of those images. Often when I'm making seamless patterns, I'll switch back and forth between the "Pan" and "Move" tools frequently.
Move all four copies of your pattern to the four corners of the blank image. The edges should match up perfectly. If you see some noticeable lines where the edges meet, then you shouldn't use that pattern. If the resulting image looks seamless to you, then it will be seamless on your Sims custom content.
If you want to save the new image you just made (the one with the four layers on top of the blank image), go to "Layers/Merge/Merge All (Flatten)." If you make this new image the same size as your old image, you'll see that the pattern appears smaller on the new image. In this next picture, I resized the new image to 200x200 pixels like the original image, so you can see the difference. The smaller print will probably work better for Sims clothing.
That's it! This seamlessly tiling pattern can now be taken into GIMP or Photoshop and used to create custom Sims clothing and items. For directions on how to do this in GIMP, please read my other tutorial, here.