Hey, welcome to the forums! There are quite a few ways to go about making the Mark VI. You could try making the armor from foam or Pepakura, which are two popular ways of armor fabrication here. If you check out "The Armory" tab on the forums here, you'll see that there are plenty of templates for the Mark VI. You'll have to look under "Halo 3" since the Mark VI armor is MAINLY the same from Halo 2-3, just with a different hand plate and some graphical changes. I'll briefly go over each method here.
1. Foam is a great way to get started, and many people here have made entire suits out of foam. Both of my suits are made from foam floor mats, the kind you can buy from Harbor Freight or Home Depot. It's cheaper than the Pepakura method, although requires some careful attention to detail. The files in the Armory can help you get started, as the ones you download should have a "foam assets" folder that is filled with the foam templates for the armor. This is a great method that I really love.
2. Pepakura is a process in which you print out the same templates as above, except this time the templates are NOT from the foam assets folder, and will have small tabs on the edges. Think of it like a large puzzle! The tabs are lined up together and assembled like a 3D model. Then fiberglass and resin are used to make the models rigid and strong. It makes very sturdy and durable armor, and looks amazing! However, it requires a well ventilated area and a respirator, since fiberglass and resin are harmful to breathe in.
Both of these methods are incredibly simplified, and require some more research before you continue!
Here is a youtube video about Pepakura basics!
Here is another for foam basics:
The tutorials on this forum are really in depth, so please don't hesitate to ask any questions! Asking the cosplayers you follow or even posting on the forums here is a great way to get in contact with people that have made a few Halo suits before!