First make sure the picture looks good size-wise on your sketch pad, including some margin room.
Next cut out the image and remove the white border so you are left with 4 or 6 borderless pages.
Next tape these together so it all lines up. Use scotch tape and tape to either the front or the back.
A great thing about the PDF from Blockposters.com is that the margins on the pages printed from the PDF insure that the whole image gets printed on your printer. Simply remove the border from all of your pages and tape them together and you are good to go!