Depending on the paper you've bought, you'll be either pasting the back of the wallpaper (traditional method), soaking the wallpaper in water (ready-pasted), or pasting directly onto the wall (new paste-the-wall papers).
Assuming you're pasting the paper, work down the length of the paper and from the centre to the edge. Don't paste from the edge to the centre as this can get paste onto the front side of the wallpaper. Once you've thoroughly pasted the paper, gently fold the pasted edges together (don't crease it!) and leave it for the recommended time as stated on the label. Some wallpapers need to 'soak' for a short period, this is to allow them to stretch before applying to the wall.