you would start with a copy of the figures , or the clothes uv template or seam guide. Those are available for DAZ products on the product page for download.

That template shows you how a painted texture will be applied to the figure. Just load it to a paint program and create an empty layer to draw your text on. delete the template layer , and then save your picture.

Apply the texture in the Poser material room . if you need to start from scratch its right click, new node , 2d , image map . click on the file name area to select your image.  Connect the image node to the Diffuse input by clicking and dragging from the top left corner of the image node to the diffuse node. The icon is plug and socket.

same thing for transparency map , except transparency is grey scale with black being transparent and white being visible. Greay shades are partial. Plug that into the transparency node .