Enabling guides to help center in Inkscape

I'm starting to learn to use Inkscape, and I'm going through this tutorial on creating a shiny clockface.

Each hour position is represented by a circle, so I want to place the circle's rotation cross mark is the absolute center of the circle, as well as put the circle at one of the 12/3/6/9 o'clock 开发者_如何学运维positions, as in this picture:

I can't figure out how to enable guides (the blue horizontal and vertical lines in the pic) to help me center and snap everything into the correct place, though.

Any ideas? (I'm using the latest version of Inkscape on Mac OS X.)

Inkscape currently (v 0.48) doesn't have any smart way of placing guides, but you can use some tricks to get around it:

1. draw any object (dot, rectangle, whatever)
2. place it where you want using the Align and distribute tool
3. then do "Object -> Object to guides".

For example, to place vertical and horizontal guides at 50% of the page do:

1. "Edit -> Guides around page"
2. Draw a rectangle so that it snaps to the corners of the guides
3. "Object -> Transform ..."
4. Select "Scale tab", enter 50% width and height, Apply
5. Move the rectangle to upper left corner of the page
6. then do "Object -> Object to guides" and you get guides at 50% of the page.

If you want to precisely move the guide, double click it.

To change the angle of the guide, hold down shift key and click and drag it with the mouse. To get 45° diagonal guides, pull them in from the beginning or end of the page ruler (i.e. from the top left, top right or bottom left corner).

Guide Lines may be what you need. A horizontal guide can be dragged from the middle of the horizontal ruler onto the drawing field. Similarly a vertical guide from the middle of the vertical ruler.