the guide must be clamped
Use a factory edge from a piece of Masonite as a guide. Make it about 8" - 10" wide if you want to run the router base against the 1/4" edge with just a straight cutter, no bearing.
Use a 1/2" MDF factory edge underneath if you are using a bottom bearing bit. Clamp them all together to avoid movement.
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)