How to create a cryptocurrency backed by an asset (say a fiat currency or physical commodity)
Value is a subjective matter and as such you can't make something be worth something else.  

As such, your best bet is to create a cryptocurrency and have all participants agree that it's worth the desired asset. 

Every other scheme with oracles and "smart contracts" and whatnots are just an iteration of the previous aforementioned social contract between the participants of the network. 

If you still want to try to develop something I'd advise you to take a look at MakerDAO and DAI from Ethereum, here. They are working on creating a stablecoin without any of the stable asset. Be advised that any people (me included) have doubts about that.