Okay, now the "how to".
1. Start with these pieces and tools - you need one set of chaos space marine legs, on chaos warriors body front, one chaos warriors body back, a base, a noise marine gun + arm, plus a chaos space marine back pack, shoulder pad (1) and head (or warriors head) - I forgot to show those last 3.
My preferred tool is a jeweler's saw (shown) as it allows for fine cuts and you can steer the blade somewhat to cut around details. Personally, its my most useful hobby tool. You will also need a hobby knife and green stuff (or other epoxy putty). You also need greenstuff shaping tools or a flat toothpick or similar object.
2. Saw off the warrior's legs just below the hanging armoured plates. You do this by first sawing down one side of the chain mail, and then making a cut across the leg until the 2 join. Then do the same on the other leg. This will leave you with the chainmail triangular "post" between the two now empty legs. Use your hobby knife to clean up the area - if the triangle is too big, you may need to trim the back portion fit the space marine legs.
3. Cut the space marine legs, at about the bottom of the crotch or a bit higher. Try to have as "flat" a cut as possible - its very easy to have it slope upwards or downwards, which isn't fatal but may make fitting the legs into the warrior's piece you had previously cut.
The seperated chaos marine legs.
4. Using greenstuff, fit a chaos marine leg into the corresponding "hole" on the warrior front. You may have to cut the leg and/or the crotch a bit to get them to fit. Once you have, some greenstuff has likely squeezed out the edges - trim it or push it back so it isn't visible, and smooth the green stuff to hide the seem between the 2 plastic parts. Then try it upright to see how it stands, move the leg into the desired position. You will then likely have to shape the greenstuff again, as some more may have squeezed into view.
5. Leave the leg to cure. Once it has, repeat with the other leg. Note, if you don't let it cure before moving to the second, its harder to ensure the leg stays in the position you want it.
6. Glue to the warrior front part (which we've been working on) to the base.
You can see that the models left leg (on our right) was cut too short, and I had to extend it with green stuff. That would have been very difficult to do had I not let the first leg cure.
7. Glue the front part of the warrior (that we have been working on) to the back part (the bit with the cape).
8. Cut just below the warriors arm back to where is joins the body. When doing all cutting, grab the torso rather than the legs.
9. Cut just beside the neck guard on the shoulder. If you can manage it, have about 1mm space between the neck guard and the saw, as otherwise you will likely mangle the neck guard a little (as I did) and it is harder to cut around the round bit of armour protecting the front should pit.
10. Saw down until the cuts join - its usually easier to see if you are looking at the back. You may have to return to the sideways cut and continue it if they don't meet.
11. If your cuts meet, the shoulder should fall off. Or, if they are close enough together, you can break it off with your fingers. Clean up the area (such as the remaining bit of the warrior arm in the photo below) with your hobby knife.
12. Repeat those steps for the other shoulder.
13. Dry fit the gun arms, then attach them to the torso using either glue or greenstuff. I prefer greenstuff, as it allows you some "flex" to pose them a bit, plus the surface from the cut is usually pretty bumpy, which isn't ideal for gluing.
14. Attach a shoulder pad of your choice - my recommendation is one without much raised ornamentation, as that will be hidden by the cloak and may make the cloak bulge strangely. Do so with greenstuff - I find glue doesn't work well between the noise marine shoulder and your standard shoulder pad.