New H1B transfer

Hi
I have applied for a new H1B(not a transfer) with a consulting company
and I just have a receipt notice from USCIS can I transfer my H1B to
another company. I have another company which is ready to transfer now.
If so, when can this transfer happen
a)ie can it happen when we just have a RECEIPT NOTICE from USCIS and
what DOCUMENTS have to be submitted?
b)or should it happen only after the H1B approval happens and you work
with the h1b sponsoring company for a while before applying for a
transfer to another company and what DOCUMENTS have to be submitted.
c)or Can it happen immediately after the H1B approval without working
for the sponsoring company and what DOCUMENTS have to be submitted?

Please reply ASAP.