I figured this might be useful to someone, and I was so pleased with how the addition of cloaks to my chaos space marines turned out I wanted to show exactly how the bits went together.
These plastic cloaks came from the Warhammer Chaos Warriors boxset (I purchased mine from Bitz Box). I used clippers to remove the top of the cloaks and then whittled the thickness at the top so that when attached to the back of the model the backpack will still fit. Finally I cut a V in the top to make a better fit. The width of the cloaks can also be adjusted by whittling or sanding the left and right edges.
The final step is to greenstuff the top of the cloak and fill the gap. Most of this area is covered by the backpack and pads so the GS work doesn’t need to be perfect. If your GS work is up to scratch it is possible to have the top of the cloak to flow over the top of the pads rather than underneath (much like on this Vulkan model).