I found a very cheep circlip plier (approx $3) and opened it up slowly so that the space was 4.5mm as stated in the attached paper.
Then carefully fitted it in one side at a time. The hardest part was in the beginning and the clip flew away a few times
Then I took a transparent plastic bag and wrapped it over the diff and my hand so it couldn't go very far
Take your time. This is an important step while building.
Happy racing!
/j0pp3 (RC-newbie)