Here you can trade players 1-for-1 or multiple (any combination of up to 3 players.)
Select your player(s) to trade then select the other teams. When done you'll see a yellow bar which denotes trade strength and basically predicts how likely your opponent will accept the offer.
After you make an offer you won't receive any notification right away. It will take some time so check your post-game reports for the updated status.
IF YOU CHANGE YOUR MIND DON'T HIT THE 'CONFIRM' BUTTON! CLICK CANCEL (BUTTON ON RIGHT THEN EXIT)
You can also add money to your offer. This will help you avoid having to offer multiple players in a deal.
At top right it displays your team funds available.
This screen is from an older version but is a good example of a trade offer from the CPU. It appeared after a game which is saying the Giants are offering me a trade.
Please be careful here! The top left button is ACCEPT TRADE and the top right button is DENY TRADE. If you click the wrong button you just bought their player!
The numbers inside the small colored boxes (e.g. 33, 32, 4) represent # of Pitchers, # of Infielders, and # of Foreigners respectively. The text inside the large black box is asking "The Giants have proposed a 1-to-1 trade. Will you accept this trade?"
(image from 2010 version)