Initial Data Loads: FI-GL, AR, AP, AA & Inventory

To do it manually you will need to enter them in through the document entry screens in each application.
GL- FB50 - Enter G/L Account Document,
AR- FB70 - Invoice , AP- FB60 - Invoice.
FA- AS01 - Asset. ( for credits use those tcodes in AP and AR instead).

For assets you will create the asset first then load data using FB50 or F-02 - General Posting.
The offset for each posting created will probably go to a conversion account which should zero out when the conversionis complete.

You can enter the inventory through a movement type transaction MB01. You could also load as an inventory update.