Shown below is the complete break down and reassembly of all un modified SKS rifles.
First, take your cleaning kit that should be located in the trap door of the buttstock, and get the punch out. If you do not have one, find yourself another punch or other strong pointed object.
Make sure your safety is on. Use the punch (or similar) and push in on the tang latch near the back of the fire control group "FCG" housing. The FCG will pop up, and then you can pull the entire assembly out.
Next pull back bolt carrier, and your magazine will come loose.
Once the magazine is pulled out, release carrier.
Make sure your bayo is extended, if your rifle has one.
Pull stock off
Next, turn receiver cover latch up...
...and then out.
Pull receiver cover and recoil spring out.
Pull bolt carrier back, and lift out. Bolt will come too.
Next rotate gas tube latch located on the rear sight block upward to the 10 o'clock position, then and lift gas tube up and out. Be careful not to rotate the latch too far forward. (Stay tuned)
The piston is located in the gas tube and will slide right out.
Put your finger in front of the rear sight block as pictured below, as the operating rod and spring will come flying out once you rotate the latch further.
Your SKS is now completely disassembled.
To reassemble, push the operating rod and spring back into rear sight block and rotate lever until it stays put.
Put piston back into gas tube
Put gas tube back on starting with gas block and push down into place, then rotate the lever back down to the locked position.
Take bolt carrier and bolt and fit them together like so:
Put into receiver and push forward. Sometimes this is easier tilting gun vertically so that the muzzle is on the floor and the receiver is pointed upwards.
Put recoil spring back into carrier squiggly side forward.
And then push until rear of spring is flush with rear of receiver.
Put receiver cover back on, make sure latch is out.
Push receiver cover as far forward as possible, push latch in and rotate downward.
Insert the fore end of the stock into the stock ferrule.
Fully seat the barreled receiver into the stock.
Insert the magazine front first, and be sure the lip goes under the retaining lug.
Pull the bolt carrier back and push down on the mag, it will now fully seat.
Make sure the hammer in the fire control group is locked in the rearward position.
Line up tabs on fire control group into slot shown.
Push forward and downward on fire control group until it snaps into place. The rear tang latch on receiver will lock into slot in fcg housing.
Your SKS is now completely assembled. You may now fold bayo back.