First, set Difficulty to Easy in Options and disable at least the Offside and Forward Pass rules. I recommend playing as All Stars - NRL All Stars, the highest rated team in the game.
On offload represents a successful, caught-in-mid-air pass to a teammate which is performed while you, as the ball carrier, are being held in a standing tackle. You can only offload if you get tackled by a single opponent and even then there are several specific tackles which do not permit offloads.
The basic strategy is to simply run face first into an opponent and just a split second before you get grabbed start mashing either pass left or pass right. The ball must leave your hands after you get grabbed and the receiving teammate must catch the ball cleanly in mid-air for it to count.
When the opponents have the ball, attempt to intercept their pass at the play-the-ball phase (this occurs immediately after a legal tackle, when the play resets). They usually pass it off to their closest teammate either to the left or to the right. Just choose a side and position one your players in the way of the pass, right in front of the potential receiver. With some luck your player should catch the ball mid-air and you can continue the offload grind. You can also attempt to strip the ball from an opponent by mashing the tackle button (but not directly from the play-the-ball initiator, that would be way too much).
Needless to say, this will take a very long time and will be very frustrating. Make sure to at least score a field goal every match in order to get wins while grinding these offloads. Once you get the hang of it, you'll average 15-20 offload per 10-minute match. Get ready to lose your mind every time the offload doesn't come out at all or comes out very weakly or the receiving teammate fails to catch the ball, resulting in a knock-on or the ball gets intercepted by an opponent.