I have done this modd allready a few times on the 55 rounds mag and they are easy to take appart as you can get the spring out before disasembling. With the 30 rounds mag this is more difficult, but i found a really easy way. I based my knowledge on well mag's.
Open it up carfully (springs tend to julp out) see that you collect all the pieces.
Drill the holes from the inside to the outside (so that you don't damage the internals), remove the drill waste, get the paracord through.
Install the spring guide (i glued it in, but don't think it is needed) and close the mag. Take the spring and feed in from the top.
Add the bb pusher (there are two theeth that guide the pusher in the mag, in the picture on the right)
depending on if you put these theeth to the right or left side of the mag will determ how far the pusher comes out. I placed them to the left which means it came out further (you can see the side that will allow this when the mag is open) this will block my nozle when the mag is empty ensuring i don't dryfire at a target.
Once the pusher is in place take the lock mechanism (you can atatch the small springs to this element up front)
and push it with the right side up into the opening.
Now with a screwdriver push down the bb pusher and push the lock mechanism in place. Voila asembled without any problem.