This is a slight update to the Version 2.0 of my guide to converting Actual Scale Space Marines (original post found here) that makes arms much more straightforward and doesn’t rely on having an abundance of terminator arms. I’ve tried to make this as easy to follow as possible, a picture says a thousand words, so they say :-p
Most of the above is pretty self explanatory. You’ll need a clayshaper/colour shaper to get a really smooth finish on each stage using Green Stuff. Note: it is best to apply too much Green Stuff and then take away the excess than not apply enough and have to add more and deal with the need to blend two bits together. In some cases you can wait for the Green Stuff to cure before cutting away excess, this will also give a much sharper finish to the edges if done properly.