I have never seen any instructions either - other than "install the clip". But this is what works for me...
On the G250 (as well as the metal cased R109 Adjustable and R156 Balanced Adjustable) you need to insert the clip in the hole (obviously) and then can usually get the clip back up over the top of the air barrel by just screwing the knob in (it works much better if it has an all metal adjustment knob) as it will push the clip up over the edge of the air barrel as it squeezes it out of the gap between the knob and airbarrel.
If you have a plastic or coated knob, and/or if the clip sets to low to roll over the edge of the air barrel when the knob is screwed in, you can lift the clip up over the edge with the tip of a screw driver by closing the adjustment knob enough to let you twist the blade of the screwdriver between the knob and clip. Be very careful though not to use excessive force as it is possible to bend the stem on the knob.
When using a screw driver, you want to be sure to insert the screw driver into the gap between the clip and knob from the side so that you are twisting against the knob (in effect pushing out on the knob) rather than from over the knob where you would then be levering off the knob and pushing it sideways - a sure way to bend the stem.
It's a small detail, but generally speaking whenever you have a choice, pins, screws and clips should be inserted from top to bottom so that if they come loose gravity holds them in place. It probably prevents them from being lost a little less often than if they were inserted from bottom to top.