Here is a video.

That shows how to solder them.

How To Solder EC3 Connectors - YouTube

Maybe it's an overkill for speaker wires, I just want to have some flexibility by having one easy access point to change from a direct line from the amp to the door speakers or passive crossovers.

I have seen some guys add similar connectors after soldering pig tails to the speaker terminals also, personally I just wanted an accessible point, in the dash or behind the Glove box that it will always be there when I need it. Of course the best way is to have the passive xovers next to the amplifier, but that is not always possible when the room is limited.

For the door speakers I crimp and solder the spade gold connectors to the wire leads and connect the spade connectors normally and use heat shrink on the terminals. Not a bad idea to just solder the wire to the speaker terminals when possible and add HS when a helper is around or extra hands are available.