How is the previous transaction hash calculated? Can anyone provide a clear example on how to calculate a previous transaction hash?

Say I want to create a new transaction, I get that the hash is calculated by doubling sha256 a message m, but I'm having a hard time understanding the structure of m.
You do not need to calculate previous transaction hash, because it is already known. 
To spend the previous output you just have to know


tx hash
output index in this tx
output value
output script


plus you should choose


fee
new address


then create some byte data and sign it with appropriate private key 

Good manual you can find here: How to redeem a basic Tx?