Using a tight zigzag stitch (I use 1.5 for the width and 0.2 for the length), sew down the right hand side of your guide line.
Test your hole with your button to make sure it's the right size. Increase the hole at the edges in very small increments if it's too tight.
Once you are happy with the length of your buttonholes, use the same zigzag along the open edges of your buttonhole, making enclosed rectangles.
Increase your zigzag width (I use 2.5 for width and keep it at 0.2 for length). Starting at the top center of your buttonhole, stitch down the right side again, using your fingers to open the buttonhole. The blue lines in the picture shows how the left side of the zig zag will fall at the outer edge of your original zigzag and the right side of your zigzag will go into the hole itself. This will bind and enclose all of your raw edges where you cut and give it a neater finish.