Creating a Value at Risk (VaR) Model for futures?

I am attempting to build a value at risk model for a variety of different instruments, but I am having some trouble theoretically understanding how futures and options fit in. I get why and how a VaR model works and have built a few on spot instruments, I just want to make sure I am taking them into consideration correctly when I make one that includes futures and options positions.

So, how do futures and options affect a VaR model? or should I essentially treat the instrument like the underlying?