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?

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?