Guides can be positioned by clicking the pointer in either the vertical or horizontal ruler and dragging a guide out to the desired position. You can use the X Y cursor position readouts 
in the bottom left-hand corner of the viewing window to accurately position the guides.
If rulers are not visible, you will have to enable by clicking the Ruler icon 
to toggle them on.
You can toggle guide visibility by selecting the View > Guides > Show menu checkbox, use keyboard shortcuts (Mac) Cmd ; / (Windows) Control ; or press the Guides icon 
in the toolbar to toggle them on or off.
You can lock or unlock all guides by checking the View > Guides > Lock menu checkbox.
Or you can display the Edit Guides Panel by selecting View > Guides > Edit... menu.
Edit... Guides Panel
You can use the 
(plus) button to add a guide, and select Horizontal or Vertical under the Type drop-down menu. Once a Type is selected, you can enter either an X or Y position for the guide, depending on the guide type.
To delete a guide in the panel, select the guide and press the 
(minus) button.
Ruler Guide Origin
To aid in visual placement of guides, the 0,0 point of the ruler can be moved by clicking in the ruler origin box in the top left-hand corner of the viewing window and dragging it to the desired position. You can reset the 0,0 origin back to its default position by double-clicking in the origin box.