It depends on your knowledge and experience of course, but perhaps learning by example is a good way to start if you're experienced C++ programmer. Since most of the cryptocurrencies are opensource, there is a lot of examples to look into. Here're few classic ones (I believe you can google much more):
https://github.com/bitcoin/bitcoin
https://github.com/litecoin-project/
https://github.com/zcash/zcash