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(Note: I'm aware my Photobucket-hosted images have been broken for a long time now. When I get access to either my backed-up files on my external hard drive OR my Photobucket account, I will try to fix them.)

Anyway, this is a super hasty tutorial thrown together to explain how to use GIMP to make tileable patterns.

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For this tutorial, I decided to start off with drawing some wintery snowflakes:
Image A: input image (not tileable)


This is just done on a 200 x 200 image by painting with a 3px circle brush using white. Draw whatever you like or import a pre-existing drawing that you would like to tile.

Of course, your input is not guaranteed to be tileable. Here's what it looks when I paint with it:


This actually doesn't look too bad, but if you stare at it long enough, you can see the edges (the seams). It looks boxy. And this is a very simple pattern -- the edges will look more noticeable with e.g. more colors.

The easy way to tile

First off, GIMP is nice because it has a super quick/lazy way of making a tileable pattern. Once I have the above original image, I can just go to Filters > Map > Tile seamless... Instantly, GIMP does some kind of blending at the edges of the image to make it so that the outer edges match up and so the image as a whole can be used as a tileable pattern.

Result:


How it looks when tiled:


Now, that's not bad. It does remove the seams and it took me very little time. However, it results in a blurry look in parts of the pattern, which may not be what you want, and it doesn't give you much control.


The harder way to tile

The manual way involves cleaning up those outer seams yourself. The way to do this is by offsetting the image by 50% in both directions. What this does is turn interior seams into exterior seams and vice versa. I'll illustrate what is going on in a bit.

But first, here's how to offset your image: copy the input image you're using onto its own layer. The easiest way to do this is by copying visible (Ctrl+Shift+C) and pasting (Ctrl+V). This will make the current view of your image as a floating selection -- click the Create New Layer icon in the Layers panel (leftmost icon - looks like a paper with a plus on it) to turn it into a new layer.

Then, while that new layer is selected, go to Layer > Transform > Offset... (Ctrl+Shift+O) and click the "By width/2, height/2" button. Make sure "Wrap around" is selected and hit Offset.

The result is this:
Image B: Offset image (offset by 50% in both directions).


Here's a more helpful diagram of what happened:
Original image (Image A) -> Offset image (Image B)
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Original image with lines overlaid -> same image after offset.

Note: The interior seams (A/B/C/D) were smooth/seamless in the original image (Image A), so in the offset image (Image B) they are guaranteed to also tile smoothly as long as we don't touch those edges.

What we can see in this offset image (Image B), though, is that what used to be exterior edges clearly have visible seams. In the lower-left part of the center is an unfinished snowflake, and on the right side, between the top half and the bottom half, the snowflake just suddenly stops and refuses to cross the border.

But now that it looks like this, we can fix the exterior edges (the parts near the center -- imagine drawing two lines like I did above -- in the offset image, Image B). And as long as we leave the old interior edges alone (A/B/C/D, which are now on the outer edges of the image), this pattern will tile nicely!

Using my brush, I fix up the interior edges:
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Now when I tile the image, it's seamless and also not blurry:


As long as you haven't messed up the exterior edges, and as long as the offset image looks seamless to you, then it will tile!


Working out the kinks

Sometimes, though, it is hard to fix the interior edges in the offset image while not messing up the exterior edges -- for example, at the top/right/bottom/left points (12/3/6/9 o'clock), the parts you're fixing and the parts you need to leave alone get very close to each other.

Also, it's hard to visualize what the pattern will look like when you tile it (you may want to add more snowflakes, or take some out, or find that the spacing is wonky, for example).

For the first thing, if you end up needing to touch the exterior edges either to fix up interior seams or because you don't like how the pattern looked, once the offset image looks seamless to your eyes, you can offset (by 50% in both directions) again and fix up the interior seams once more. Again, as long as you don't touch the exterior edges, if the offset image looks fine to you, it's guaranteed to tile!

For the second thing, I suggest previewing what your pattern will look like when tiled by keeping open a large document (with dimensions big enough to get in several repetitions of your pattern in both directions). Go to the offset image you're fixing and copy what it looks like (Ctrl+Shift+C). Then go to your large document and select the Bucket Fill tool (Shift+B). Under tool options, select the third option (Pattern fill) and under the pattern selector, choose the first option (Clipboard Image). Then click in the document to flood fill with your in-progress pattern.

Whenever you want to preview again, copy visible from your work in progress, clear the large document (using Edit > Clear or the Delete key), and paint bucket again.