How to customize checkout and order

Hi,
I have project that need customization on checkout and order process.
1. I need to add order confirmation to vendor first before customer can pay the order, so customer don't have to do refund if vendor cannot process their order.
2. I need to add ability for vendor to add shipment information such as : shipment company/provider, shipment method, tracking number, tracking website URL, price.
basically the order process will look like this :
customer fill the address and do the checkout ->
vendor/admin get the order and confirm with shipment information and cost or cancel the order ->
if canceled customer will get notify why the order got canceled
if confirmed customer have to do the payment, I already create the plugin for the payment, ->
vendor/admin will get notify and have to send the goods and update the order status ->
customer can trace the order using shipment information provided by vendor/admin ->
customer have to do the confirmation that he/she already received the order  ->
order finished.

I don't know how and where to do the customization with plugin.
Any idea how and where i should start?

Thanks.

This will be challenging.  You will probably need to allow the order to be initially placed with some dummy shipping and payment method because the vendor can only see placed orders.  Also, you will need to consider whether to allow the cart to contain items from multiple vendors.

New York wrote:
Correct, i get similar suggestion from Yidna, will try that today. I'll update later.

Nalysa wrote:
update :
still working on using action filter i get from this post : https://www.majako.net/how-to-create-a-custom-action-filter-in-nopcommerce-v4/
to bypass shipment and payment on checkout from custom plugin.