ofc im to lazy to write this myself,
so i'll just post the link(s) that explain a great lot of it
http://www.soasewiki.com/index.php/Trade_Port
text-based
http://www.soasewiki.com/index.php/TradeRoute2_LargeMap
picture-based
note that all this is is written pre-entrenchment, same rules apply, you just need 2 build starbases in neutral gravity wells, or places where you are not able to build a tradeport without a starbase
S_o_L