Google Richard Rosenman. He offers several free plugin filters that I find extremely useful, in this case a tiler plugin. The one thing you have to make sure of is that your top and bottom and side to side match reasonably well. There are probably lots of tutorials if you google paint tileable textures, but really, it's more something that comes with practice.

What you have to watch out for are large, distinctive features in your images, which when tiled will repeat annoyingly and make it obvious that it's a tiled texture.

Talent is God's gift to you. Using it is your gift to God.