Tools required: 5mm allen key
1. Loosen slightly
Loosen the two side bolts slightly (see fig.2 below). These will have been screwed in slightly pre-transit to ensure they don't move around in the box and damage the bike.
2. Straighten the handlebars
The bars can now be straightened, whilst the wheel remains stationary.
Tip: Stand with the bike and saddle between your legs, as if you're riding the bike yourself, but without sitting on it. From this angle, you'll be able to see if the handlebars are straight and comfortable.
Positioning: the wheel should already be with the front brake towards the front of the bike and so that the cables are bunched out in front, see below.
An easy way to position the bars is pictured below:
3. Tighten the screws
Tighten the side bolts you loosened in step 1. These need to be quite firm. Tighten until you are unable to tighten any more.
Tip: This is usually easier to do with two people. One holding the handlebars straight, and the other tightening the bolts.