There is no right or wrong way to play this game and so ultimately, it's entirely up to you how you tackle each level. However, in the interest of going fast, here are some tips:

This strat is only recommended on Easy difficulty. As the mission starts, spawn in the hangar of Echo Base. Enter the nearest Snowspeeder and wait for your teammates to spawn around you. When they have spawned, use the directional pad to order one of them into your Snowspeeder with you. Take off and fly to the Forward Camp on the other side of the map.

Once there, land, order your teammate out, exit the vehicle yourself, order your teammate to stay put and capture the command post together. You may have to fight for it, if you do, your teammate can assist you (Note: By the time you get there, most of the Imperials will have moved away from the command post, allowing you an easier time to take it). Once captured, get back in the Snowspeeder and order your teammate to do the same. (Note: If your teammate died, it's a reset, as your teammate is vital for the next part). The reason you take the Forward Camp before taking on the Walkers is because it will give the Imperials something else to focus on rather than your other existing command posts. There will (hopefully) also be less resistance at this command post when you return to it later.

Take off and fly to the AT-ATs and begin to use the tow cable to take them down (This is where your co-pilot comes in). Try and take out the Walker at the back first to avoid being shot by the other Walker. If you get really good RNG, there's a chance your team will have taken out one of the Walkers before you, though this does not always happen of course. Once both AT-AT's are down, fly back to the Forward Camp. Most likely the Imperials have re-captured the Forward Camp or are at least in the process of taking it back. Sometimes, they won't have touched it, which is rare, but is the best possible outcome. Regardless though, you will still have to defend it.

Once again as before; land, order co-pilot out of vehicle, exit the vehicle and capture the command post again. This will be a slightly harder fight than the previous one as there are likely a group of Imperials that spawned at the AT-ATs and are most likely assaulting the command post. Capture and defend the Forward Camp with all you've got! Once captured, the victory timer should be counting down by now. Continue defending until the timer reaches zero. (Note: there is a small possibility that the Imperials might have taken one of your other command posts, if they have this is also a reset. The reason is because the AT-ATs are on a timer, and in the time it takes you to take back the Forward Camp and an additional command post, it will be too late and the AT-ATs will have respawned).

If you've done done everything correctly, you will have just beat Hoth: Echo Base in under 4 - 5 minutes. :)