Ok. I have acquainted myself with a player who has recently set up a colony in my system. We agree that a trade would be helpful for the both of us. I am unclear as to exactly how I'm supposed do this.

trade rates are noted in the "Trade" section. you agree, then you load up your cargo ships, he sends his to you using a "transport" mission, and you do the same.

So if you are giving him 3k metal and he is giving you 2k crystal (just a basic example) just send it to him. If either of you receive your shipment and decide to get crazy and stop your shipment, then you can be punished with a game ban for cheating (milking/pushing)

That is basically all there is to it, but go to the trade section and read. There are more indepth discussions and explanations on this subject.