Another useful tool is the ability to rotate guides in Illustrator. You can do this by first creating a horizontal or vertical guide on your Artboard. Next, select “R” and then click anywhere on the guide. This should create a retical point. Finally, click and drag anywhere on the Artboard and watch your guide rotate on the axis point you created. You can hold down Shift while dragging and reach 45° easily.

Short: Create Guide > “R” + Click Guide > Click/Drag Off Guide = Rotate Guides