It's pretty straightforward.
1) Get their friend code (it's a chain of three sets of four digits) and add them to the friends list on the home screen of your 2/3DS. Give them your friend code and have them do the same.
2) Boot up your preferred version (X/Y can trade freely with ORAS), load up your game, and turn on your WiFi on your device.
3) Once they come online, they'll show up in the top list on your lower screen (the friends and acquaintances list). Tap on them and select trade.
4) Complete the trade. NOTE: If you are receiving a pkmn from someone, you'll still have to put up an unwanted pkmn, as all trades have to go both ways.