Apply two thin coats of primer to the surface that you are painting, allowing each coat to dry fully.
Apply a light brown glaze to the surface, brushing it on randomly.
Drag a clean brush through the wet glaze to begin establishing a grain direction (typically vertical). Wipe off the bristles after every stroke. Use this technique until the entire surface is lightly streaked.
Allow the glaze to dry for 24 hours.
Apply a darker glaze to establish the heart grain. This can be done by hand with a small brush, or with store-bought grain stamps or rollers. If you are unfamiliar with using these tools, practice on a piece of scrap wood before moving on to your project. Photos of oak grains (or pieces of actual oak) can be very useful guides for this step.
Allow the glaze to dry for 24 hours.
Apply a coat of urethane or varnish to protect your work. Allow it to dry fully. Drying times will vary with the product used.