What keeps the armor from sliding down or off

I've just joined up and I'm excited for this summer where I'll be able to get the materials and time to begin my trek to spartan-hood among many of you veteran pepakura artists. As I day dream about it all, I started wondering a few things including how a shoulder piece or bicep piece may stay on. Same with a thigh piece. As much as I would wish i could slip my arm in a new shoulder piece and it would just perfectly meld itself to whatever under suit I use in the future, I know this is unlikely. I've heard of people using hooks and straps too keep pieces attached, is this common? Is Velcro common? Also, any suggestions on black under suits that could mimic that of spartans rather than a simple morph suit? One last question; I wanted to make a shoulder piece then add a Scout shoulder onto that as would be seen in game so that I could make other shoulder armor pieces and add them on and take them off when I wish; is this possible? What do i do to keep the two shoulder pieces (the one that wraps around the upper arm then the armour piece itself) connected?

All help would be much appreciated, thanks for your time.

Velcro.....lots and lots of velcro lol. There are many ways to attach pieces to the undersuit so they'll stay, or not even attaching them at all. I've even started using zip ties for certain pieces, but Velcro is definitely the most commonly used method.

I just use buckles and straps and just clip mine together but its odst and it look the part.

Velcro and cloth straps and buckles that match your undershirt are what you'll need.

You can also use snaps and magnets in addition to velcro.

Hey there, I'm a noob at this too but I know what you mean about the morph suits they seem very simple... Now I am going, on my project, with a compression shirt and pants for the main bit of the under suit. I'm of an average/athletic build so I think it will look decent this way. And looking at videos like the Spartan Ops series from Halo 4 and such you can tell that these suits aren't baggy or non-form fitting at all. And the ones I found on Amazon are very cost effective with good reviews. So I am spending about $30 all together on it all. As opposed to other options.