Place the butt stock between your knees with the recoil spring assy pointing away from you, and push the circlip off using two screwdrivers if it is the "C"-type circlip (6). If it is a snap ring you simply use the snap ring pliers. In stead of placing it between your knees you could place the stock in a vice but I don't recommend this as it would be easy to damage the stock.
After removing the circlip (6) remove the locking ring with spring (5) and the clamping lever (4).
Remove the protective cover (3) from the back plate (1) by pressing the tabs in with a pair of pliers and pulling the cover off.
Now you can slide the butt stock (7) with the protective cover (3) and the gripping lever (2) off from the back plate (1). Removing the gripping lever from the guide rails might be a little tricky, place the gripping lever in the guide rail locking recesses used to lock the stock in the extended position. Then wiggle it a little while carefully bending the guide rails very slightly apart, and the gripping lever should slide off.
How to assemble the retractable stock.
Slide protective cover (3) onto butt stock (7).
Insert gripping lever (2) in the recesses and push butt stock to half length into the back plate (1).
Press the protective cover (3) into the back plate (1). Attach clamping lever (4) to the buffer housing so that the two lugs engage properly in the gripping lever (2).
Insert locking ring with spring (5) so that the end of the tension spring engages in the bore of the locking ring (5).
Insert circlip (6) or snap ring(6).
Pre-tensioning the spring in the clamping lever.
Retract butt stock all the way up to stop.
Swivel clamping lever to the left.
Swivel back clamping lever all the way to the stop.
Press ratchet into the notch of the locking ring and swivel back clamping lever.
Repeat this process until the clamping lever is under spring tension.Locking ring with spring (5), detailed view.
*Please note that there are some types of stock that might disassemble in a slightly different way, but I think the basic principle is the same. Examples are the HK33/93/94/MP5 stock. I haven't tried to disassemble any of these yet.
If you assemble your retractable butt stock incorrectly it might collapse on you while shooting, causing severe injury.
If you choose to proceed, you do this on your own risk, and if you somehow manages to break or otherwise ruin your stock, well that is your problem. If you are not 100% sure that you can manage to do this, don't do it, or pay someone competent to do it for you.