Welcome to this tutorial! This shouldn't be too hard for you to do, so lets get started!
What we need first is a good initial texture. I shall use the following, but anything goes.
Do make sure you're using a reasonably good quality though! Otherwise it might look horrible in game.
Open your image up in photoshop. What you want to do now is to make sure that the width and height of the image are both even. Luckily my texture already is, but if yours isn't you can easily adjust that with:
Image >> Canvas Size
And enter two even numbers (decreased only one or two in size!!) for instance 763x157 would become 762x156.
And now for the real magic! Do this:
Filter >> other >> Offset
This should prompt you with a new window. At width you enter the value of 50% of the width of your file, same goes for height. So in my example we'd have to enter: Horizontal: 350
Also select the "wrap around" function.
Now remove obvious blemishes using the Healing Brush Tool(J).
I will not go into detail of using that tool, as it is listed in the help file of photoshop.Here is what I've got up to this point:
If you did everything right up to this point you should now see a vertical and a horizontal line through your texture. No problem! Time to fix that up the same way as in step three, with the Healing Brush Tool.
If you're done with that do the offset filter again. Just press control + F for that. If you see a cross again, you want to remove it again. If you don't see it, go on to the next step. If you do, repeat this step until you don't.
Well! You're done. You now have a texture that can easily be tiled! Congratulations!
Feel free to contact me if you have any questions.