You can add guide lines to the drawing area by clicking and dragging on the rulers with the selection tool.
For a horizontal guide, click any where on the top ruler and drag the guide line downwards.
For a vertical guide click anywhere on the left ruler and and drag the guide line to the right.
Once a guide has been added, you can move it in the usual way with the selection tool.
If the grid snap is set, the line will snap to the grid.
To remove a guide, drag it onto the left or top ruler as appropriate.
Options
The "Guides" entry on the top menu allows you to set various options for the guides.
Lock guides
This prevents the selection of any existing guides. To unlock the guides, click the same menu entry again.
If the guides are lock, you can still add new guides, but once added you can move them until the guides are unlocked.
Clear guides
This remove all the guides.
To remove a single guide, drag it onto the left or top ruler as appropriate.
Show centre mark
This shows a cross in the centre of the page. To hide the centre mark, click the same menu entry again.
Guide colour
To change the guide colour click on one of the palette entries.
The new colour is applied to all existing guides and any new guides.