How to maintain selections for products in selection ids; APO DP

Hi,

I am new to APO DP. I want to do forecasting , can anyone tell me how to exclude , include my products under specific planning area from forecasting process.

Is that to be done by maintaining SIDs for the respective material??

Please let me know how exactly this SID works and how to get into that step by step.

Regards,

RKShukla