It's a very simple process and Psyonix has created a safe system to Rocket League Credits ensure players won't get tricked and robbed. Cross-platform trading is enabled so players old and new will have endless options.
To initiate a trade, players need to invite their desired trade partner to a party. Once joined, there will be an 'Invite to Trade' option. Once both players accept it will take you to a screen showing your entire inventory.
Select the items you want to offer and display them for trade, as will your partner. Once terms have been agreed players can accept and the items will be swapped. There is www.lolga.com a game chat available throughout the entire process to ensure players can safely trade.