With the Magnet tool (U) you can define Power Guides manually or edit links created with Glyph > Add Power Guides.
Adding Element Guidelines»
First make sure glyph and element guidelines are visible (View > Glyph Guides).
Select the Guides tool in the Toolbar and add an element vertical guideline: press and drag with the ++Alt-Shift++ keys down. Position the guideline on a vertical stem. Ctrl-click (or right-click) on it and select Vertical in the context dropdown menu. Similarly, you can also position the guideline on a horizontal stem, select it and choose Horizontal in the context dropdown menu.