Okay, no pictures right now but I'll try to explain it as best I can. It can be a major PITA.
First, get some parts ready, put the cocking tube into the gun and orient it so the roll pin hole is lined up with the access hole in the top of the gun (the hole you used to punch the pin out)
Put the spring into the little relief cut in the charging handle, the spring should be pointing to the 7 or 8 o-clock position.
NOW, before doing anything else get the roll pin, and put it part way into the access hole in the gun's receiver. DO NOT put it all the way in, just enough so the cocking tube can't move.
You will want to put the charging handle in now, this is tricky. You want the spring to get compressed so it's pointing at the 9 O-clock position when in the gun. This is where the roll pin being in is helpful, you want to keep the pressure BACK on the charging handle so the spring stays compressed, this may take a couple times (and keep a hold on that roll pin, if it falls into the gun you have to take it all back apart). Once the charging handle is close to the right spot, start pushing DOWN on the roll pin so it gets into the spring hole, then tap it gently with a hammer to get it started in the charging handle's hole.
After it's started you can CAREFULLY start using an 1/8th inch (or up to 4mm) roll pin punch to start driving the pin back in. Be VERY careful with this step, as you can't push it up, you have to take it all the way out and begin again if you push it in too far.
You want it to just get hidden inside the cocking tube, so it doesn't rub or scratch the inside of the cocking tube support portion of the receiver.