The calculation is simple.
Value of your investment minus value of your final capital when you cash out equals what you have to put in your tax return.

If your hidden question is : Is there a way to hide my gain.
The answer is : Yes you can try, but every one of your trades are stored on exchanges, and if in 2 years your country look into it, and see that your strange increasing bank account value doesn't seem legit, you are completely fucked, because everything you earned might be lost in the fees of your cheating.
Worse than that, you mught have spent it already, and not be able to pay the fees anymore.
You are dead. Game over.