Visit a local lumberyard and explore the grains and textures of the wood that they offer. Decide if you prefer tight grain with knots or rough-hewn, splintered wood. Consider your audience. For example, a logo for a company selling finely crafted furniture should not have a rough-hewn look, but a logo for a tree-removal service might call for such a design.
Snap a photo of the wood grain that you wish to use as an overlay on your text. The grain should run horizontally if you want to create continuity between letters, but should run vertically if you want to emphasize individual letters.
Open your graphics-editing program -- such as Paint.NET, Adobe Photoshop or Paint Shop Pro -- and paste your digital image into the "Background" layer.
Create a "New Layer" in the "Layers" menu. Name the new layer "Box Layer." Create another layer and name it "Text Layer."
Place a black rectangle on the "Box Layer" using the "Rectangle" tool in the "Tools" menu.
Enter the text that you wish to overlay with wood grain in white using the "Text" tool in the "Tools" menu.
Select the white text with the "Magic Wand" tool in the "Tools" menu and press the "Delete" key. With your selection still active, move to the "Box Layer" and then press "Delete" again. This will allow the wood grain to show through your text. Repeat for all other letters or symbols that you wish to have the wood grain.
Add shading to the edges of your lettering to suggest depth. Lower the wood grain's opacity with the "Opacity" option in the "Layers" menu to create subtle tones with your "Paintbrush" tool.
Save a copy of your image as a file that is native to the program you are using -- such as a Paint.NET or Photoshop file -- and as a JPEG, PNG or TIF.