Hello Invisible SAP,
SAP Help along with accessing APO IDES is the way to go.
Since you are more focused in DP and SNP Here is the link to SAP Help.
Demand Planning Process - SAP Library
Supply Network Planning - SAP Library
Besides, you can look for any opportunity to work on APO project preferably under the guidance of experienced APO Consultant(mentor)
Hope this will help. Good Luck...!!!
Thank you
Satish Waghmare