Transaction E41B - Create Portion
SISU > Scheduling > Schedule Master Record > Portion
Create your Portion name, assign an
- Parameter Record,
- Billing Period Date,
- Allocation Date,
- Scheduled Bill Date
In the Period Characteristics, Months = 3
when creating the Portion also assign a Factory Calendar.
For Dynamic scheduling, set the Dynamic scheduling field.
Transaction EA43 / E43A
SISU > Scheduling > Schedule Master Record > Dynamic Schedule record
E1DY - Create/Replace
If starting from scratch, choose the Factory calendar & execute for required date range.
Otherwise, use a reference portion to copy the schedules.
It sounds to me, that you will need to start from scratch.
I suggest practicing in a sandbox.
Yell out if you still have any specific questions.
Alternatively for completeness....
For Non-Dynamic scheduling, set the Dynamic scheduling field
Transaction EA43 / E43A
SISU > Scheduling > Schedule Master Record > Schedule record > Create
generate the schedules for the date range required.