This tutorial is still a work in progress. However, once you get the general idea, it shouldn't be too much trouble for you to make the rest.
Step 1, preparing the torso: You will need a Cadian torso. All the details on the torso need to be removed, except for the collar. Be ruthless! Cut off all the armour plates and fabric. Once these have been sliced off, smooth it down using a file of by gently running the edge of the blade across the surfaces. Try to get a smooth, torso-like shape.
Step 2, adding shoulder straps: Slice a number of thin strips of the thinnest styrene sheet you can get hold of. I guess thick paper would do as well, but I'm not sure how you'd glue it down. The strap should be a just over 1mm wide, and around 20mm long. Test it by wrapping it round the sholder. Once you're happy with the length, glue it down, making sure it doesn't obscure the flat arm positions on the sides of the torso.
Step 3, adding shoulder straps: Repeat step 2 on the other shoulder.
Step 4, button lace: To keep thing simple and in keeping with the uniform for my regiment, I'm using squared ended button lace. You could do it rounded, pointed, bastion looped or any other shape. I've gone for simple straight ended. Cut out four small oblongs, and test their size before gluing. Once you are happy with their size and shape, glue them down, being careful to leave a gap between the 1st and third loop from the top. This is where the cross strap from the shoulder straps will lie, and we need to do as much as possible to reduce the 'height' of the straps from the torso, else they will get in the way of the arms.
Step 6, adding the cross strap to the shoulder straps: Cut a 1mm thick strap that will reach from one shoulder strap to the other, and glue it in place so that it lies in the gap you left where the second of the button laces should have gone.
Step 7, adding buttons: Find a thin bit of styrene dowel, and cut it into thin discs. Stick these in a vertical row onto the button lace. If you can by pre-cut 1mm-ish discs, please let me know from where! You may wish to leave the buttons until after the cross belts are added.
Step 8, Crossbelts: Each crossbelt needs to go from one hip, over the opposite shoulder, and back to the hip. I do three belts - one for a white bag for food etc (left hip, right shoulder), one for the cartridge box (right hip, left shoulder) and a final one for the bayonette scabard (left hip, right shoulder again). On this final belt, you need to add a small oblong or square of styrene sheet to be the breast plate. You could do a forth, a thin one for the water bottle over the bayonette belt and breast plate, but for the sake of thickness I've chosen not to do so.
Note: Don't worry about how the back looks. It will be covered over by the Trotter knapsack (backpack) and other equipment. Once the glue is dry, you could even try to file off the straps.