It is actually not that hard, but the first time might be confusing.
A few things to remember:
You need to open a channel, create two containers and set the source and the destination container.
After this init store the dict as the input.
The input dictionary needs to be set exactly as per GDL manual stated. If you have not enough, or too much (!) keys inside the dict the whole thing will fail.
The Parameters for the thing you want to do are stored with "preparefunction". The actual operation is a "callfunction".
Retrieve the polygon (also a "callfunction").
For anything more paste your code here and tell us what exactly fails.
