The key insight you are missing is that to specify relative coordinates you can use
++. For example, from
(current page.south east) if you want to draw a line 2 cm up from this point, simply write
(current page.south east)--++(0,2cm). This means, move 2cm in the
y direction from the previous coordinate, with no change in the
To get back across the page to coordinate 2cm above
(current page.south west) I've used the
-| command, which tells TikZ to find the right angle intersection point between the two coordinates. Note there is an opposite version of this called
|- which would in this case return the coordinate
(current page.south east).
So all in all, the path you want for this rectangle is
(current page.south west) -- (current page.south east) --++(0,2cm) -| (current page.south west) -- cycle.
So in one exercise you've now learnt two really useful drawing commands in TikZ. There are of course a number of other ways to do it, one of which is using
rectangle as shown in John Kormylo's answer.
Additionally, If you want this 'footer' to appear on every page, then you can use something like this.
I've put the TikZ code in a command I've named
\Footer and then used the package
atbegshi as described in TikZ header and footer across top and bottom of bulletin/newsletter page.
\fill [remember picture,overlay,blue] (current page.south west) -- (current page.south east) --++(0,2cm) -| (current page.south west) -- cycle;