The tcp header follows the IP header. You'll find the sequence number in the tcp header (google tcp header).
What exacly are you trying to create? If you want to examine data and then alter it/add to it before it reaches the internet then you could do that by creating a proxy server that your application connects to and the proxy server receivs the data and changes it/adds to it then sends to the server. This would avoid having to deal with raw packets.
