Newby questions

I want to add lines to a flat surface at precise locations. Research on the internet seems that the colour paint icon is the one to use. Unfortunately, when I click on this the background grid disappears so I can only guess where to draw the line.
What is the best way of doing this?

Also, I want to look down at the top surface without any offset - is there a way of doing this without rotating the screen image?

Any help greatly appreciated.