Remove the blade from the planer and fit it into the honing guide. Make sure the guide is holding the blade bevel-side down. Set the guide on a perfectly flat surface and adjust until the entire bevel is contacting the surface.
Oil the whetstone with tapping oil, which is designed to aid in the removal of metal. Other lubricating oils can hinder the sharpening process by protecting the metal surface you're trying to wear away.
Use only the coarse side of the whetstone if there are nicks in the blade you need to remove. Hone the blade using a pushing motion, not a pulling one. Pulling will make a burr on the blade edge that will feel sharp, but will wear away after the first cut.
Switch to the fine side of the whetstone only after any nicks are completely gone from the blade edge. Push the blade over the whetstone, using even pressure so that the blade does not rock. If the blade rocks, it could make a curved edge on your planer blade and you will have to reshape it on the coarse stone again.
Check the blade periodically to make sure the bevel is being maintained. Turn and tilt the blade in the light, watching the light reflect on the bevel. It should appear perfectly flat.