Sorry in advance for the rough pics - my photography rig is at work, which is not a convenient location to work on a tutorial!
Tau Jump Pack Tutorial
List of pieces needed: 1 XV8 Crisis Suit Fusion Blaster, 1 Firewarrior Backpack, 2 Firewarrior Photon Grenades, 1 Firewarrior Shoulder Pad, 1 Space Marine Terminator Stormbolter (AOBR)

Step 1: Cut the back segment from the Fusion Blaster (where it connects to the arm). Next, cut off the barrels section the forward end. I cut just a hair behind where the vents end.

Step 2: On the small barrel piece, trim a small groove along the back (vented) edge - this makes it look like a separate segment once it is re-attached to the jump pack. On the Fusion Blaster body, cut one side of the vented section at an angle, starting by the back edge of the vents until it almost meets the top edge of the other side. (I do this cut conservatively, cutting at more acute angles, over the course of several cuts, so that I don’t mess it up!)

Step 3: Trim the front of the ammo clip from the storm bolter. (Cut deeper than you need to, and clean the edge back to straight afterwards - this piece tends to be fragile, and can easily curl under an aggressive cut, leaving you with an unusable air intake.) Then cut a groove to fit the air intake along the angled edge of the jump pack from step 2. At this point, I glue in the air intake and add the exhausts to the bottom edge of the jump pack.

Step 4: Trim the sides of the Shoulder Pad until it can sit flat on the back (vented side) of the Jump Pack. Cut the handle from the backpack, and then cut it in half (width-wise) so that it will sit flush with the other side (air intake side) of the jump pack. Next, Glue these pieces to the jump pack. I try to align these pieces with some of the existing lines on the jump pack, to give it a feel of intentionality.

Step 5: Take the two Photon Grenades, place them side by side (with the detailed side facing down) with their bottom edges facing away from each other. Then cut them at an angle, from about halfway through the length of the body to the edge of the top (rounded) section. (This gives the impression that the top exhaust tube is longer than the bottom one.) Note - Make sure you cut the second set of grenades in the opposite direction as the first! Glue these exhaust tubes onto the sides of the jump pack, and it is finished! (with the exception of drilling out the exhaust tubes...)

I will add a tutorial on creating the gun later!
(edited for smaller pics)