See this CRIMPER tool on TOURATECH-USA.com
There are some VERY IMPORTANT diagrams at the bottom of this page.
It is possible to assemble this AMP connector with simpler tools.
The diagrams at the bottom of the photo are necessary.
NOTE which side of the terminal is "UP" relative to the housing. Slide the terminals into the housing 100% until they click in place. THEN push the red "keepers' in while watching the wire, to make sure the wire (+seal) is not backing out as you push the keeper in. Once assembled, it is almost impossible to un-assemble.