for you should have a look at this wiki page. It describes the configuration and usage of snappy in general.
In the examples directory you find the snappy_subset.py script which shows the usage of an operator.
In the end you need to call GPF.createProduct with the name of the operator (Maximum-Likelihood-Classifier) the parameters and the source product.
You can find out which parameters are available if you call on the command line
gpt Maximum-Likelihood-Classifier -h
or you configure the operator in the GUI and then select from the menu File / Display Parameters…
if you search her in the forum for GPF.createProduct you will get several additional examples.