First, start out by gluing the Chaos Marine's legs to the base.

Match up two torso halves, spread some glue along the edges, and press them together.

Attach the torso to the legs. You might consider how you want the arms positioned beforehand.

Attach the grenade/pouch bit to the belt of the Chaos Marine.

Now you can attach the arms in a suitably dramatic pose.

Next, glue on the Chaos Marine's head. Make sure it matches the rest of the pose.

Next, choose your shoulder pads and glue them on one at a time.

Lastly, glue on the back pack. You now have a fully built Chaos Marine.