Some times you can found a cross stitch fractions during embrodery, work them as follow: start on a corner of a square and finish the stitch in the center of the fabric square. in the picture from left to right, a 3/4 stitch, 1/2 stitch and a quarter stitch.

all the stitch fractions are used to improve the quality of the design, rounding edges and angles to make them more natural.
It si possible to find stitches fractions in different colors for example one color for the quarter stitch and another one for the remaining half stitch in the same square.

to make out diagonals you may embroider stitches diagonally, working them in a sense and in the other, alternatively, as shown in figure (the green lines are the rappresentation of the back of the canvass)

To complete the diagonal simply embrodery back the cross stitches paying attention to pass under those stitches you embroider in the "back" sense, to make the whole stitches orientation looks fine. (orange lines are the rappresentation of the back of the canvass)