Standing the walls
You will need a bit of a hand to stand the walls.
The next part will take three people (just to make things easier) but only for a few minutes.
Lift the rear wall and position it in place.
Have one person balance the rear wall, while the other two people lift a side wall into position.
Use a couple of clamps to hold the two walls together.
You can now make the third person redundant. Send him home before he eats all the cake!
Lift the other side wall into position and hold it in place with another couple of clamps. Likewise with the front wall.
Fixing the walls together
Nail at the corners through the front and rear wall studs into the side wall studs.
Use 4" (100mm) galvanized flathead nails. Four at each corner.
Also nail the bottom plates to the sub-floor where possible.