To start with, grab the pieces you need - the backpack tanks, the flamethrower, a head, a torso, a pair of legs, and one of the extended arms with a grip clutched in the fist.
Glue the legs and torso onto a base.
Next, glue the tanks on his back. Take note of the small pipe-tap bit jutting out on one side. There is a hole in it.
Next, glue on the flamethrower arm - there is a pipe sticking out the rear of this model piece - you want to glue it to the tap hole on the backpack, and align the shoulder with the torso, to glue it in the correct position.
Next you glue the other arm on. The grip clutched in his fist slots into a hole on the flamethrower, just like the pipe and the tap did. You want to glue the grip into that hole, and align the shoulder with the torso, for this arm to be correct.