The interlock sequence can be interpreted normally in one of three ways. I will list and explain all three interpretations of how to tackle this achievement that you will find when scouring the internet.
For those of you who are visual learners, I have linked my video at the bottom of the solution. It is my first YouTube video for a game ever, so be kind with your criticism.
Do remember all games have glitches and/or hidden rules.This achievement can be slightly temperamental. Sometimes the interlock sequence is what is covered in this solution, sometimes it doesn't matter at all the order you enter. Simply try to unlock the interlocks across from each other every time until the achievement pops.
Since HW:DE is a Play Anywhere title, if you have a Windows PC that can handle HW:DE, some people have been having success getting glitched achievements to unlock on their computer.
Rules of the Interlocks
1. There cannot be any non-human units near the interlock. Covenant, Flood, or Sentinels. I personally had the sentinels near one preventing the interlock from being activated when going for this achievement.
2. It's advised you have a unit near each interlock. It should work without one, but better safe than sorry.
Method 1. Found in the Xbox 360 solution. Personal favorite. ↓
Basically number the interlocks from 1 to 6 with the interlock closest to your base being 1. From there number the remaining interlocks counterclockwise around the center of the map. Once you're setup to open the interlocks the sequence will be...
1, 4, 2, 5, 3, 6
Method 2. The Clock ↓
This one can be more confusing for some, but it is still very simple. When you start look at the interlocks on the hexagon as if it was a analog clock represented with even numbers. Your starting base is at the bottom of the clock at 6 o'clock. So the sequence would go as...
6, 12, 4, 10, 2, 8
At the time I am posting this solution Tyrael has this as his solution, but I found a forum post by Chill Penguin X on the official Halo Wars website from 2009 explaining this method, though he got 2 and 8 backwards.
http://www.halowars.com/forums/thread/485877.aspx Method 3. 1 to 6 with Directions (Also used in video below.) ↓
This one can get a little confusing as it uses the 1 to 6 numbering, but the interlocks are numbered in the sequence you have to open them. Now you can go in the sequence as explained in the directions you can find in the Halo Nation wiki, but someone made a picture to help that I linked below.
"
http://i43.fastpic.ru/big/2012/0919/c2/37729971640bc2ef73aec...If you want the directions they are...
1. The door just North West of your Starting Base.
2. The door directly opposite of Door 1.
3. The door to the east of Door 1.
4. The door directly opposite of Door 3.
5. The door to the north of Door 3.
6. The door directly opposite of Door 5.
http://halo.wikia.com/wiki/Thinkin%27_about_My_Doorbell