Pretty straightforward achievement.
First off, from the main menu, select "Mission Editor", then "Create".
From there, phase 1 "tiles" is to build the map using the 2d shapes, RB/LB to change type, LT/RT to change different piece, D-pad to rotate, and A to place. When you are happy with your map (make sure you leave to some open bits for Genestealers to come from), push Y to move to phase 2, "interactive objects" such as doors, marine/genestealer entry points.
Use back to give your map a name, and RT to give it an objective. Survive is probably the easiest, as it does not involve any more moving parts, such as exit, and just needs entry points for Marines and genestealers.
When you are happy, push start, and select "publish". Achievement unlocked!
If it doesn't let you publish, check by pushing start that there are no red x's which will prevent you (eg no objective chosen/no spawn points, etc), and just correct your map.
IF you publish, and there are no cautionary orange comments as well, then you will also unlock
as well
Good luck!!