When it comes to starting a cross-stitch, especially a big one, everyone has a different strategy for tackling the piece.
There is the focused approach, where a central object is sewn first followed by another object until the final picture is formed. You get to see the fruits of your labour and can abandon the rest at any time.
Some like to sew from each of the 4 corners and gradually build the final piece from there. This is a good technique if you are doing a giant signboard as it can get tiresome counting and moving across the fabric.
My favourite technique is to sew by colour, then do a tick back on the codes so I know when I am done. I have also found this to be efficient for pieces with lots of colours and where colour mixing is required.