It can be a **** job because of it's location. I do it by holding down the cap firmly, then I push in the middle of the retaining clip which effectively lengthens the clip and then push the top of the clip into it's retaining slot in the cap. It is also a hell of a lot easier if the bonnet is removed.
Hi, I usually get some strong string and make a very long loop on the end.
(around 12” of loop)
Then place the cap on and place the loop in the hook on the end if the retaining clip. Pull up on the loop and the retainer will click into place on the cap.
Then cut one side of the loop and remove the string.
You may have to use 2 loops.