As soon as you can start your camp fire.
There is a cooldown before you can upgrade it again.
Sometime around the first night, you should be able to upgrade your camp again.
Do this as soon as possible.
Do this one more time before day 3 and you will get two free walls.
Sometime Night 2 you should be able to barely buy the upgrade before morning.
You do not have to finish building the walls to get the achievement.
You should not build walls before it gives them to you for free, or you can mess up getting it ;D
Goodluck!