Hi Tom

I looked everywhere for a guide, never found one though.

Best place to start is :
1. Remove barrel - just grub screws.
2.Remove end charging cap
3.Remove hidden grub screw that is located in the bottom of front barrel strap
4.Undo cylinder - standard left hand thread. It may be super tight and need a clamp. Set aside
5.Tip up stock and tap butt to get the exhaust valve to drop out.
6. Undo action stock screw and trigger guard.
7. Lift action up and out of stock.

I am sure you can figure the rest out. Valves are from a titan I think. Exhaust valve is made from nylon or similar and on my Alros rifles they are all slightly different.
I would source another barrel if it was mine as you may loose some fps with the short barrel unless the valve has been modified.