(See here if you've no idea what this is about)

Okay, here goes. You'll need the following:

Insert the hook like so. The half bushings are important, because they align the hook correctly and also provide friction to keep it in.

Rotate the trigger whilst inserting the hook further. Keep pushing until it's in as far as it can go.

Done! If it's in place properly, the tip of the hook should be touching the trigger and you should be able to knock this about without anything coming loose.

And just note that the half bushings should be as far up as possible like they are there, as they have bad grip if they're sitting lower down in the barrel.

The more you know!