The trick with rulers is to remember that the needle is 1/4" inside the ruler foot, so you have to aim your ruler to allow for that 1/4" discrepancy. Once you've got that figured out, it gets a lot easier.
For me, I keep my "ruler" hand on the ruler and on the fabric to guide it and my other hand on the fabric on the other side of the needle and use them both to guide the fabric and ruler. Try not to press the ruler hard against the ruler foot, but just use it as a guide.
Watson