I want to begin with the T-codes when uploading demand forecast to APO and R3.

1 before uploading forecast data to APO, the data should be csv format, not excell one. Below is an example,

Purpose: upload demand forecast to temp cube in Administrator workbench

P.S. Administrator workbench is one kind of basic data-base. Many kinds of basic data can be updated here.

Steps:

1) choose temp tube

2) upload demand forecast to chosen tube

Before start recording the T-codes, we need to figure out the structure of demand planning in SAP.
In my company, the process of demand planning can be divided into 2 parts,
1 demand forecast in APO
Basically, I treat APO as a forecast management tool now. Maybe one day I can use it to do deployment or some other things; I still have a long way to go.
Firstly, sales data and forecast history are timely updated. By using certain statistical model in APO, you can get a draft demand forecast. Then you can adjust the forecast by SKU according to the sales target and promotion plan.
2 MPS, production scheduling, MRP (material requirements planning) and CRP (capacity requirements planning) in R/3
When the demand forecast is finalized, it will be transferred to R/3. Next step is MRP and CRP.
Also, we have BW report, which is a great assistant to me. We have daily inventory report and weekly forecast accuracy report to follow up the business status. These reports can help us to adjust the forecast and production very convenient.