1. Know C++
this allows =>
2. Understand the algorithm of the program. What it does.
3. Know VB6
these allow =>
4. Write VB6 code that does the equivalent operations as the C++ version.
You don't, is probably the answer. If it is more than a page or 2 ... the languages are too different, you would be better off to start over and use the same algorithm and data structures (loosely). Literally every line of code would require changes, most lines the changes would be substantial. And a literal line by line translation would probably yield sub-par basic code.
One solid option would be to compile the C++ into a library in visual studio and call that from VB.