First, you’ll want to arm your uplinks. The game points them out to you; just go to those locations and it’ll tell you what to do once there. After you arm the uplink, defend it! It’s best if you use a bubble shield here, since the enemy team will try to stop you the second things are armed.
In the second phase, you’ll want to start firing at the AT-AT. This is where the Ion Shot comes in handy, but you can make use of other stuff—like aircraft and smart rockets—too. Just know that you alone probably can’t do a ton of damage, so coordination will be key.
In the last phase, you’ll want to make use of your snow speeders, as they can take down ATs pretty quickly. Of course, this is easier said than done...even with your entire team playing the objective, it’s still really easy to lose. Hopefully this gets rebalanced a bit once the game actually launches, because right now, it kind of sucks to play as Rebels. So don’t feel bad if you lose, Rebels.