Open the image or use an existing image to be the source material. You can also apply some effects and color work to the design. Go to the select menu and select all or use the selection tool / marquee in the toolbox. Go to the patterns panel which can be found via the window menu and media control panels. Go to the right menu side. Capture pattern command. Set the name etc as required
Once you have defined the item, you can use it in features of the application such as clones and original luminance settings and settings in fill command as well as brush strokes, apply surface texture, express texture, adjust dye concentration, custom tile, distress, apply screen, blobs etc
You can find the panel in the 'media control panels' section of the window menu in Painter. The right side menu comes with a range of commands such as rename as well as create an icon for it etc. The key one for capture is the capture command. The define and make fractal commands are of interest, in particular, as the fractal option can be used to create some very unique tiles
You can panel does have a few minor additional features such as name (as well as a very small preview which is pretty standard for the application. You can generate a regular tile (and most of the time I go for that option) but you can also apply a horizontal shift or vertical shift to the tile and you can then use the bias to shift the alignment of the tiles. Useful for brick fills etc. The great thing about the capture command is that you can select different areas of an image or perhaps apply effects etc. You can use the tool to create a snapshot of all kinds of images and type and effects to create a huge selection of tiles for use with brush strokes and clones etc. As mentioned, unless you start with an image that is seamless, you probably won't get a seamless tile but the tiling feature is great with either. You can also use the result of any generated tile as a new image which can then be re-selected and used as a new source for a tile. Literally billions of unique designs / artworks can be create using these tools.