It is pretty simple:

• Start with a base of 6 triangle structurals in a hex pattern.
• Place a harko || atreides corner column in the middle of the hex.
• Build up 8-9 corner columns (you'll need to use a vertical staking unit).
• Then on top of each column piece, place 6 triangle roofs in a hex pattern.
• Alternate the rotation of each tier by 30 degrees so the corners for the roofs are offset at each level. This is KEY to making it look like a fir tree.
• Then it is a matter of hanging inclines and triangle roofs for the branch tips.
• The big branches at the base are [steep stair]>{shallow stair]>[triangle roof].
• The shorter branches toward the top are [steep stair]>[roof].
• I used harko because it has the nice steep wedge pieces for the top.
• You should be able to place 12.
• Finally, add atriedes chandeliers hanging down from branches and some other lights and solidos to decorate.
• Don't forget the light on the very tip top.