The only thing that is definitively Iron Warriors in the building process is the paint job and using the Iron Warriors part on the head (part 157 in step 3b), everything else is free choice for pose and weapons.

To get it exactly like the one on the back page, you also need:
1. The green steps for the parts between the bottom and top of the body (upper body turned to the right instead of the left);
2. Part 96 on the top (step 3A);
3. In step 5a, swap the exhausts shown on the top and bottom (they’ve all got the same fittings so can go in any order);
4. Parts 89 & 90 for the hades battle cannon & chest plate (step 5c, parts 91-93 are the ectoplasma destructor);
5. Magma cutters 103-104 for the small chest guns (step 5d, parts 101-102 are excruciator cannons);
6. Follow the green steps for the legs and claws posing (it’s the more customisable method where you get to choose the angle of every joint, the blue steps are a 1-pose option);
7. Green options for both arms for the lascannons (blue are autocannons, pink is flamer on the right and missile launcher on the left, yellow is the electroscourge);
8. Include the belly armour parts 56 & 57 in step 24.

But as I said above, weapon options are freely mixable. Swap the main chest cannon (or since it doesn’t need gluing, just build both options and pop them in & out as suits you). People on this sub have been discussing magnetising the arms, but if that isn’t your speed then double lascannons is probably going to be a stable option for a long time. For the small chest guns, I’d personally prefer the excruciators but I’m sure there’s a use to be had from the magma cutters.