To create a new guide, simply drag your cursor from the ruler areas on the left, top, or corner of the screen. This creates vertical, horizontal, or diagonal guide lines, respectively.
Guides can also be helpful if you want to draw horizontal, vertical, or diagonal lines. Drag a guide out first, then select the Pencil tool, and draw your line directly on top of the guide. You will see the words "Handle to guide" appear as your line is snapped into place.
Double click on your guide to open up a new window where you can change the angle of rotation. This will enable you to draw a straight line at any angle you want. (The current version of Inkscape also allows you to change the color and label of guides, as seen in the image below)
You can also manipulate the angle of rotation by holding the Shift key while moving your guide.
When you are finished with a guide, you can remove it by dragging it off of the screen, or by hovering over it until it turns Red and then pressing the delete button on your keyboard.
Removing Guide Snapping
Sometimes, guide snapping can be annoying. There are situations where you may not want things to snap to a guide. (I encounter this when I use my guides as page margins, or when placing parts of my diagram really close to a guide). In this case, we can temporarily disable guide snapping by using the icons on the right side of the screen:
In particular, the topmost icon will turn snapping on and off. I find myself clicking this button a lot when building diagrams:
Creating Page Margins:
One of my favorite things to do with Guides is to use them to create page margins. To do this, you will first need to set up your desired page properties. Go to File --> Document Properties on the main menu:
Now, go to the Page tab, and set up your page size and default units. I set my page size to US Letter and default units to inches:
After your page properties have been set, drag out four guides (two horizontal and two vertical) to be used as page margins. Double click on each guide to bring up a new window where you can input numbers for your X and Y axes:
My default units are in inches, and I like to create 1 inch page margins. I set the vertical guides to X:1 and X:7.5, and the horizontal guides to Y:1 and Y:10.
In a future post, I will talk about using guides as a way to pinpoint a new center of rotation. This trick is very useful when you want to create pattern repeats without having to manually place them by hand.