The vast majority of your armor in the Mark VI will simply rest on your body.
Your shins, forearms, boot armor, and chest and back plate will not require attachment to the under suit, especially not for Haloween. Forearms rest against your gloves/hands and your shins rest on top of your boots. Your chest and back will be connected to each other and simply rest across the chest/shoulders.
Your Thighs, shoulders, and the Cod plate and posterior armor are the pieces that will require any real level of strapping or attachment.
The cod and posterior armor should be mounted to a belt system. This can be with an actual belt or with nylon strapping, or any other strapping material, that you fashion into a belt. You can either attach the belt directly to the cod and posterior armor, or you can fashion "Belt loops" to the inside of the armor, either with soft or rigid material. On my Halo CE Mark V I fashioned rigid belt loops out of scrap plastic that I glued to the side of the armor and feed a belt through.
For the Thighs the best system it a strap that goes from the thighs that attaches to the same belt used for cod and posterior, or a secondary belt worn under that cod and posterior set up. Once again on my Mark V the straps are permanently glued to the inside of the thighs. The top of the straps are sewn into a loop that the cod and posterior armor belt feeds through.
For the shoulders you have several options. You can either attach them to the chest and back armor with a strap, or you can attach them directly to the under-suit. I'll use my Mark V as an example a final time. I use a set of automotive coveralls as my under-suit, since the Halo CE Mark V under-suit is more ambiguous. I have snaps on the inside of the shoulders and corresponding snaps on my coveralls. I don't actually recommend the coveralls for a Mark VI since they don't match the look of the in game under-suit, but the same principle applies. You can attach velcro or snaps to both your armor and whatever under-suit you plan to use. I find that attaching directly to the under-suit gives you the greatest range of motion.
You can attach the shoulders to the chest with a strap, so that they hang directly off the chest and back and turn it into basically a large T-Shirt. Straps can be permanently mounted, or made to be removable. I would recommend removable for ease of transport and storage. If you go Removable, plan for two points of failure on the strap system, like have snaps and velcro, or two patches of velcro that way if only one spot fails, you don't immediately lose your piece. And if you use velcro go for the wide industrial style.