how to make a trainer?

[SiNTAX]

Originally Posted by ArC

.. you put a jmp to your own code there (which can be situated in a cave for example) which writes the memory address to some freespace within the exe.

If you inject your own DLL into the executable, then you don't need to find 'free space' in the exe to put your code.
As for..

Originally Posted by ArC

Mondern games are very complex so they use dynamically allocated memory

I think making trainers in the old days (e.g. Amiga), was way more difficult than it is now. Today code is always loaded at the same address, and all code is usually present from the start.
In the days of Yore.. you had to patch track loaders because each level used different code or was loaded to a different address. And ofcourse you had no breakpoint on write access capability to find that all important SUB #1 instruction :-)
But then.. it made it more fun to do..